Use ActivityAware to get the Activity context

After the changes to support Android V2 embedding, the plugin is set up in `onAttachedToEngine` and the Application Context is not an instance of Activity.

When this context is eventually passed to `setAppId`, we end up with the current activity is null which leads to some problems. This is only a problem on the first run of the app.

This commit implements the `ActivityAware` interface and in `onAttachedToActivity`, updates the context to the new activity (https://docs.flutter.dev/development/packages-and-plugins/plugin-api-migration).

In addition, use `activeContext()` instead of `context()` for the registrar for Flutter's v1 embedding to get the application context. Doing some testing shows the former gives, for example, the `MainActivity` while the latter gives `FlutterApplication`.

Author: nan-li

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

@@ -3,6 +3,9 @@
 import android.annotation.SuppressLint;
 import android.content.Context;
 
+import io.flutter.embedding.engine.plugins.activity.ActivityAware;
+import io.flutter.embedding.engine.plugins.activity.ActivityPluginBinding;
+
 import com.onesignal.OSDeviceState;
 import com.onesignal.OSEmailSubscriptionObserver;
 import com.onesignal.OSEmailSubscriptionStateChanges;
@@ -42,6 +45,7 @@ public class OneSignalPlugin
         extends FlutterRegistrarResponder
         implements FlutterPlugin, 
         MethodCallHandler,
+        ActivityAware,
         OneSignal.OSNotificationOpenedHandler,
         OneSignal.OSInAppMessageClickHandler,
         OSSubscriptionObserver,
@@ -96,12 +100,29 @@ private void onDetachedFromEngine() {
     OneSignal.setInAppMessageClickHandler(null);
   }
 
+  @Override
+  public void onAttachedToActivity(@NonNull ActivityPluginBinding binding) {
+    this.context = binding.getActivity();
+  }
+
+  @Override
+  public void onDetachedFromActivity() {
+  }
+
+  @Override
+  public void onReattachedToActivityForConfigChanges(@NonNull ActivityPluginBinding binding) {
+  }
+
+  @Override
+  public void onDetachedFromActivityForConfigChanges() {
+  }
+
   // This static method is only to remain compatible with apps that don’t use the v2 Android embedding.
   @Deprecated()
   @SuppressLint("Registrar")
   public static void registerWith(Registrar registrar) {
     final OneSignalPlugin plugin = new OneSignalPlugin();
-    plugin.init(registrar.context(), registrar.messenger());
+    plugin.init(registrar.activeContext(), registrar.messenger());
 
     // Create a callback for the flutterRegistrar to connect the applications onDestroy
     registrar.addViewDestroyListener(new PluginRegistry.ViewDestroyListener() {