Add support for modifying ResTable on fly.

This change allows to either delete or add asset path after
a ResTable object has been created for a given AssetManager.
In past, the lack of such support forced us to implement
update configuration functionality suboptimally, by recreating
new AssetManager object from scratch on theme change.
With this change, we can simply remove the "old" theme asset path
and add the new asset path. We also need to make sure the Resources
object nukes internal caches.

CR:JoshG.