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

image_editor/android/build.gradle.kts

@@ -16,6 +16,17 @@ subprojects {
     project.layout.buildDirectory.value(newSubprojectBuildDir)
 }
 subprojects {
+    // gallery_saver_plus hardcodes compileSdk 31, which its dependencies no
+    // longer support. Register the override before forcing :app evaluation so
+    // it runs after each library script has set its own value.
+    project.afterEvaluate {
+        if (project.plugins.hasPlugin("com.android.library")) {
+            val appSdk = (project(":app").extensions.getByName("android")
+                as com.android.build.api.dsl.ApplicationExtension).compileSdk
+            (project.extensions.getByName("android")
+                as com.android.build.api.dsl.LibraryExtension).compileSdk = appSdk
+        }
+    }
     project.evaluationDependsOn(":app")
 }