fix: make every Android app build again

- Add the missing flutterEmbedding v2 meta-data to 8 manifests that still
  used the removed v1 embedding
- Drop stale package attributes from covid19_mobile_app debug/profile
  manifests that conflicted with its namespace
- Override gallery_saver_plus's hardcoded compileSdk 31 in image_editor so
  its dependencies resolve (documented AAR metadata check now passes)
- Document that the three Firebase apps need a developer-provided
  google-services.json before they can build

enabling_splash_screen/android/app/src/main/AndroidManifest.xml

@@ -7,11 +7,6 @@
     -->
     <uses-permission android:name="android.permission.INTERNET"/>
 
-    <!-- io.flutter.app.FlutterApplication is an android.app.Application that
-         calls FlutterMain.startInitialization(this); in its onCreate method.
-         In most cases you can leave this as-is, but you if you want to provide
-         additional functionality it is fine to subclass or reimplement
-         FlutterApplication and put your custom class here. -->
     <application
         android:name="${applicationName}"
         android:label="enabling_splash_screen"
@@ -34,5 +29,11 @@
                 <category android:name="android.intent.category.LAUNCHER"/>
             </intent-filter>
         </activity>
+    
+        <!-- Don't delete the meta-data below.
+             This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
+        <meta-data
+            android:name="flutterEmbedding"
+            android:value="2" />
     </application>
 </manifest>