Integrate Loader support in to Activity/Fragment.

Introduces a new LoaderManager class that takes care of
most of what LoaderManagingFragment does.  Every Fragment
and Activity can have one instance of this class.  In the
future, the instance will be retained across config changes.

Also various other cleanups and improvement.

Change-Id: I3dfb406dca46bda7f5acb3c722efcbfb8d0aa9ba