Android: Resolve request permission when already true

Problem:
If permission is already enabled, the native call to `OneSignal.getNotifications().requestPermission(fallbackToSettings, Continue.with(...)` never suspends and the Continuation code block never runs. As a result, we would not be able to resolve the promise over the bridge.

Solution:
Before calling that method, do a permission check and return true. Opt to check the permission in Android instead of resolving early in Dart because the permission boolean may not be initialized correctly yet when requestPermission is called soon after initialization.

Author: nan-li

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

@@ -78,6 +78,12 @@ else if (call.method.contentEquals("OneSignal#addNativeClickListener"))
 
     private void requestPermission(MethodCall call, Result result) {
         boolean fallback = (boolean) call.argument("fallbackToSettings");
+        // if permission already exists, return early as the method call will not resolve
+        if (OneSignal.getNotifications().getPermission()) {
+            replySuccess(result, true);
+            return;
+        }
+
         OneSignal.getNotifications().requestPermission(fallback, Continue.with(permissionResult -> {
             replySuccess(result, permissionResult.getData());
         }));