Update add to app code to run on modern versions of agp and gradle. (#2330)

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add_to_app/plugin/android_using_plugin/app/src/main/AndroidManifest.xml

@@ -1,6 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<manifest xmlns:android="http://schemas.android.com/apk/res/android"
-    package="dev.flutter.example.androidusingplugin">
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
 
     <application
         android:name=".MyApplication"