Remove dependency upon FragmentActivity

The FragmentManagerImpl is intimately tied with a FragmentActivity. In
many cases, we want to be able to create / manage Fragments outside of
a FragmentManager. This defines a FragmentController interface that can
be used by any class to host Fragments.

Bug: 19569654
Change-Id: I6816a5c1815122d206062b9f4584ad460b3d41dd