Performance improvements in AssetManager

Change the implementation of getLocales() to iterate the set of
configurations using a templated method, instead of using the result
of getConfigurations().

Also remove the check for AndroidManifest.xml when adding an asset path.
This is unneccessary.

Bug:28625993
Change-Id: I16de5da598d0c371421d1dc8eee054dce9baf53a