Add MediaProjection APIs.

The new MediaProjection infrastructure allows the system to hand out
tokens granting the ability to capture the screen's contents, audio,
etc. at a granular level. It's intended to be used both for screen
casting, via the cast APIs, as well as screen sharing via third party
applications.

The screen sharing case is implemented, but all of audio capturing
is still forthcoming.

Change-Id: I4b24669bed7083e11413c10ed8d6b025f5375316