[Impeller] Clarify where to put the metadata in the manifest. (flutter#54814)

I made the mistake of putting it under the activity tag and was surprised it didn't hold.

Author: chinmaygarde

impeller/docs/android_validation_layers.md

@@ -17,7 +17,8 @@ flutter/tools/gn \
 ```
 
 Then adding the following field to the
-`android/app/src/main/AndroidManifest.xml` will turn them on:
+`android/app/src/main/AndroidManifest.xml` under the `<application>` tag will turn
+them on:
 
 ```xml
 <meta-data