AccessibilityService.java

@Override
    public void onAccessibilityEvent(final AccessibilityEvent event) {
       ...
         UsageAccessHelper.ActivityData realActivity = appStatsHelper.getCurrentActivity();
         realActivityName = realActivity.getActivityFullName();
         checkIfPictureInPicture(realActivityName);  
    }

private void checkIfPictureInPicture(String activityName) {
        if ("com.android.systemui/.pip.phone.PipMenuActivity".equals(activityName)) {
            notificationService.fireNotificationAsync(Notifications.PICTURE_IN_PICTURE_MODE_ACTIVATED);
        }
    }

AppStatsHelper.java

...
@Nullable
    @Override
    public ActivityData getCurrentActivity() {
        try {
            long ts = System.currentTimeMillis();

            UsageEvents queryEvents = usageStatsManager.queryEvents(ts - STATS_INTERVAL, ts);

            String activityName = null;
            String packageName = null;

            int eventCount = 0;

            while (queryEvents.hasNextEvent()) {
                queryEvents.getNextEvent(eventOut);

                if (!TextUtils.isEmpty(eventOut.getPackageName()) && eventOut.getEventType() == UsageEvents.Event.MOVE_TO_FOREGROUND) {
                    packageName = eventOut.getPackageName();
                    activityName = eventOut.getClassName();
                }
                eventCount++;
            }


            if (TextUtils.isEmpty(packageName)) {
                return null;
            }
            if (eventCount == 0) {
                logger.debug(LOG_TAG + "usage stats contains no events for last %d ms", STATS_INTERVAL);
            }

            return new ActivityData(packageName, activityName);
        } catch (Exception e) {
            logger.error("[UsageAccessHelper50] cannot query usage events: %s", e);
            return null;
        }
    }
    ...
    @ToString
    @AllArgsConstructor
    class ActivityData {
        public String packageName;
        public String activityName;

        public ActivityData(String fullActivityName) {
            try {
                String[] data = fullActivityName.split("/");
                packageName = data[0];
                activityName = data[1];
            } catch (Exception e) {
                packageName = "";
                activityName = null;
            }
        }

        public String getActivityFullName() {
            return TextUtils.isEmpty(packageName) || TextUtils.isEmpty(activityName) ? "" :
                    packageName + "/" + activityName;
        }
    }

FakePictureInPictureActivity.java

/**
 * This Activity does nothing except closing itself. Used to close Picture-in-Picture window opened
 * by another app.
 */
public class FakePictureInPictureActivity extends AppCompatActivity {

    public static Intent getStartIntent(@NonNull Context context) {
        return new Intent(context, FakePictureInPictureActivity.class)
                .addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    }

    public static boolean isRunning() {
        return isAlreadyRunning;
    }

    private static boolean isAlreadyRunning = false;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        FrameLayout root = new FrameLayout(this);
        root.setId(R.id.nationaledtech_fake_picture_in_picture_container);
        setContentView(root);
    }

    @Override
    protected void onStart() {
        super.onStart();

        isAlreadyRunning = true;
    }

    @Override
    protected void onStop() {
        super.onStop();

        isAlreadyRunning = false;
    }

    @Override
    protected void onResume() {
        super.onResume();

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O
                && getPackageManager().hasSystemFeature(PackageManager.FEATURE_PICTURE_IN_PICTURE)
                && !isInPictureInPictureMode()) {
            try {
                enterPictureInPictureMode(new PictureInPictureParams.Builder().build());
            } catch (Exception e) {
                MobivementApplication.getMobivementContext()
                        .getLogger().error("[FakePictureInPictureActivity] Cannot enter PiP mode: %s", e);
            }
        }
    }

    @Override
    public void onPictureInPictureModeChanged(boolean isInPictureInPictureMode, Configuration newConfig) {
        super.onPictureInPictureModeChanged(isInPictureInPictureMode, newConfig);
        if (isInPictureInPictureMode) {
            try {
                moveTaskToBack(false);
                finishAndRemoveTask();
            } catch (Exception e) {
                MobivementApplication.getMobivementContext()
                        .getLogger().error("[FakePictureInPictureActivity]Cannot exit PiP mode: %s", e);
            }
        }
    }
}

PictureInPictureActivationListener.java

@RequiredArgsConstructor
public class PictureInPictureActivationListener implements NotificationListener {
    private final Context context;
    private final Logger logger;

    @Override
    public void onNotification(String category, Bundle extras) {
        if (!Notifications.PICTURE_IN_PICTURE_MODE_ACTIVATED.equals(category)) {
            return;
        }

        logger.info("[PictureInPictureActivationListener] picture-in-picture detected");

        if (isDefinitelyHasPictureInPictureApi() && !isOurPictureInPictureActivityRunning()) {
                logger.info("[PictureInPictureActivationListener]" +
                        "picture-in-picture is not allowed at the moment");
                ContextCompat.startActivitн(context, FakePictureInPictureActivity.getStartIntent(context), null);
        }
    }

    private boolean isDefinitelyHasPictureInPictureApi() {
        PackageManager packageManager = context.getPackageManager();
        return Build.VERSION.SDK_INT >= Build.VERSION_CODES.O
                && packageManager.hasSystemFeature(PackageManager.FEATURE_PICTURE_IN_PICTURE);
    }

    private boolean isOurPictureInPictureActivityRunning() {
        return FakePictureInPictureActivity.isRunning();
    }

}