Fixed missing OneSignal.add*Observer calls

* MainActivityViewModel has the interfaces implemented already but was just missing the add calls to hook them up.

Author: jkasten2

Examples/OneSignalDemo/app/src/main/java/com/onesignal/sdktest/activity/MainActivity.java

@@ -18,6 +18,9 @@ protected void onCreate(Bundle savedInstanceState) {
         setContentView(R.layout.main_activity_layout);
 
         viewModel = new MainActivityViewModel();
+        OneSignal.addPermissionObserver(viewModel);
+        OneSignal.addSubscriptionObserver(viewModel);
+        OneSignal.addEmailSubscriptionObserver(viewModel);
         viewModel.onActivityCreated(this)
                 .setupInterfaceElements();
     }