clearAll notifications on Android if count zero when in background (#2521)

Author: chriscant

docs/PAYLOAD.md

@@ -60,6 +60,8 @@ The following flowchart attempts to give you a picture of what happens when a pu
 - If the app is running in the background the push plugin then checks to see if `content-available` exists in the push data.
 - If `content-available` is set to `1`, then the plugin calls all of your `notification` event handlers.
 
+> Note: if `count` is given as `0` then all notifications are first cleared: always on Android, and if the `count` has gone down on iOS
+
 ## User clicks on notification in notification center
 
 - The app starts.

src/android/com/adobe/phonegap/push/FCMService.java

@@ -329,6 +329,10 @@ private void showNotificationIfPossible(Context context, Bundle extras) {
       Log.d(LOG_TAG, "count =[" + badgeCount + "]");
       PushPlugin.setApplicationIconBadgeNumber(context, badgeCount);
     }
+    if (badgeCount == 0) {
+      NotificationManager mNotificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
+      mNotificationManager.cancelAll();
+    }
 
     Log.d(LOG_TAG, "message =[" + message + "]");
     Log.d(LOG_TAG, "title =[" + title + "]");