Allow multiple Resources associated with an Activity

Previously it was assumed that one Resources object per Activity would be fine.
This proved to be incorrect, as methods like Context#createConfigurationContext()
expect to create a new Resources object that gets updated whenever the Activity Resources
get updated.

To fix this issue, there exists a master override configuration for each Activity. Subsequent
Resources created for an Activity will be based off that master override configuration, and
will be updated accordingly.

Bug:27644297
Bug:27604953
Change-Id: Idb3d851f55ee7981eec4e3fe28e2ea8d55f1192d