First draft of Scenes & Transitions feature

This checkin has preliminary API (in flux, definitely changes still
to be made) and implementation for a new "Scenes & Transitions" feature.
The current implementation allows you to define different Scenes
(via layout resource IDs or callbacks) and Transitions to be used when
changing to those scenes. By default, scene changes will use AutoTransition,
which generally does the right thing.

There are no overview docs or tutorials yet. The best way to learn how things
work is to see the code for the various tests in
frameworks/base/tests/TransitionTests.

Expect the API to change. Expect the implementation to change (mostly to add
more functionality). Expect bugs, but tell me if things do not work
as expected.

Change-Id: Ib025a9f565678b225afa4759325cf6d496cc7215