Let android create template have an explicit flutter splash screen flag (#11622)

* Let android create template have an explicit flutter splash screen flag

* minor language tweak

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

@@ -23,6 +23,13 @@
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
             android:hardwareAccelerated="true"
             android:windowSoftInputMode="adjustResize">
+            <!-- This keeps the window background of the activity showing
+                 until Flutter renders its first frame. It can be removed if
+                 there is no splash screen (such as the default splash screen
+                 defined in @style/LaunchTheme). -->
+            <meta-data
+                android:name="io.flutter.app.android.SplashScreenUntilFirstFrame"
+                android:value="true" />
             <intent-filter>
                 <action android:name="android.intent.action.MAIN"/>
                 <category android:name="android.intent.category.LAUNCHER"/>