PM: Load all splits when parsing an app manifest

Previously the base APK would be considered without
any splits loaded into the same AssetManager. This would
prevent splits from overriding attributes in the
AndroidManifest.xml, such as enabled state based on
version, etc.

This CL loads all APKs in the cluster into a common AssetManager
from which the base AndroidManifest.xml is parsed.

Bug:17006358
Change-Id: Ib88096c49d0c4f743b7e6ba0921251459bee107c