feat(android): add AdMob App ID to AndroidManifest.xml

- Add AdMob App ID metadata to the AndroidManifest.xml file
- Use test AdMob App ID for now (replace with real ID later)

Author: fulleni

android/app/src/main/AndroidManifest.xml

@@ -38,6 +38,11 @@
         <meta-data
             android:name="flutterEmbedding"
             android:value="2" />
+
+        <!-- TODO(fulleni): Replace with your real AdMob App ID -->
+        <meta-data
+            android:name="com.google.android.gms.ads.APPLICATION_ID"
+            android:value="ca-app-pub-3940256099942544~3347511713"/>
     </application>
     <!-- Required to query activities that can process text, see:
          https://developer.android.com/training/package-visibility and