Resolve request permission call when permission already exists

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 early.

src/index.ts

@@ -413,6 +413,10 @@ export namespace OneSignal {
       if (!isNativeModuleLoaded(RNOneSignal)) {
         return Promise.reject(new Error('OneSignal native module not loaded'));
       }
+      // if permission already exists, return early as the native call will not resolve
+      if (hasPermission()) {
+        return Promise.resolve(true);
+      }
 
       return RNOneSignal.requestNotificationPermission(fallbackToSettings);
     }