[BKNDLSS-28250] Push notifications with Android 12 (#519)

Author: kao78

src/com/backendless/push/BackendlessFCMService.java

@@ -178,7 +178,8 @@ private void fallBackMode( Context context, String message, String contentTitle,
 
     Intent notificationIntent = context.getPackageManager().getLaunchIntentForPackage( context.getApplicationInfo().packageName );
     notificationIntent.putExtras( newBundle );
-    PendingIntent contentIntent = PendingIntent.getActivity( context, notificationId * 3, notificationIntent, 0 );
+    PendingIntent contentIntent = PendingIntent.getActivity(
+            context, notificationId * 3, notificationIntent, PendingIntent.FLAG_IMMUTABLE );
 
     notificationBuilder.setContentIntent( contentIntent )
             .setSmallIcon( appIcon )