Cache per-Activity Resources objects

Each Activity has a Resources object whose underlying state changes
when a configuration change occurs, both global and for that activity only (multi-window).
Views and other clients of the Resources object can safely cache it knowing that
they always have a reference to the most up-to-date resources.

This applies to Resources.Theme objects as well.

Bug:26854894
Change-Id: Ifd3da5a1086e17f747dfc265a447fe92a3a5c692