Clean handler before set

* Avoid handlers being set more that once, native side will keep reference too all handlers set

Author: Jeasmine

android/src/main/java/com/onesignal/flutter/OneSignalPlugin.java

@@ -160,6 +160,12 @@ private void setAppId(MethodCall call, Result reply) {
   }
 
   private void addObservers() {
+    // Clean observers before setting, avoid being call twice
+    OneSignal.removeSubscriptionObserver(this);
+    OneSignal.removeEmailSubscriptionObserver(this);
+    OneSignal.removeSMSSubscriptionObserver(this);
+    OneSignal.removePermissionObserver(this);
+
     OneSignal.addSubscriptionObserver(this);
     OneSignal.addEmailSubscriptionObserver(this);
     OneSignal.addSMSSubscriptionObserver(this);