docs: Improved description of "package" attribute (manifest).

The description of the "package" attribute for an Android
application's <manifest> element now states more clearly that the
Android package name and the Java package name(s) used in the app's
classes do not need to be the same.

Bug: 2735276
Change-Id: Ifa30615c1cca5f6f19e4d1d0c071e4e256b3151b