[AChoreographer] Add private api that decouples from ALooper.

For ease of use by HWUI, we'll get rid of the reliance of a thread-local
ALooper:
* Rather than using AChoreographer_getInstance to access a thread-local
AChoreographer instance, the caller will be expected to create and
destroy its own instnace.
* For use by HWUI's own looper instance, _getFd() will expose the
underlying file descriptor from which events can be listened. The
corresponding choreographer combined callback can be called to flush
pending display events.

Bug: 136262896
Test: builds
Change-Id: I94f102da22974cbad37f21a68d2f04ac0cec5c78