Replaced FirebaseInstanceId shadow

* FirebaseInstanceId no longer exists in the latest Firebase versions,
shadowing FirebaseApp as a replacement.
* We are shadowing initializeApp isntead of getToken as initializeApp
in the latest firebase version causing an expection to be thrown on a
background thread which fails the tests.

Author: jkasten2

OneSignalSDK/unittest/src/test/java/com/onesignal/ShadowFirebaseApp.java

@@ -0,0 +1,20 @@
+package com.onesignal;
+
+import android.content.Context;
+
+import androidx.annotation.NonNull;
+
+import com.google.firebase.FirebaseApp;
+import com.google.firebase.FirebaseOptions;
+
+import org.robolectric.annotation.Implements;
+
+@Implements(com.google.firebase.FirebaseApp.class)
+public class ShadowFirebaseApp {
+
+    @NonNull
+    public static FirebaseApp initializeApp(@NonNull Context context, @NonNull FirebaseOptions options, @NonNull String name) {
+        // Throw simulates Firebase library not bundled with app
+        throw new RuntimeException();
+    }
+}