Remove FlutterApplication from app templates. (#64742)

packages/flutter_tools/templates/app/android.tmpl/app/src/main/AndroidManifest.xml.tmpl

 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="{{androidIdentifier}}">
-    <!-- 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="io.flutter.app.FlutterApplication"
         android:label="{{projectName}}"
         android:icon="@mipmap/ic_launcher">
         <activity

packages/flutter_tools/templates/module/android/host_app_common/app.tmpl/src/main/AndroidManifest.xml.tmpl

@@ -8,13 +8,7 @@
     -->
     <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="io.flutter.app.FlutterApplication"
         android:label="{{projectName}}"
         android:icon="@mipmap/ic_launcher">
         <activity