Move the guts of Resources to ResourcesImpl

In order to satisfy the requirement that clients can
cache a Resources object when a configuration change happens,
we move the caches and all other method bodies to ResourcesImpl.
These can then be swapped out for the correct version when needed,
while allowing clients to keep holding the existing Resources references.

This is part 1 of 2 CLs. The next one will do the actual switching of implementations
based on configuration changes for multiwindow.

Bug:26854894
Change-Id: I41156194a3541e59053b4048c3a15981c7d8a506