SF: Creating a basic Scheduler class.

Added a flag that can be turned on at runtime, and turns on the new path.
Added a basic Scheduler that encapsulates SF and App thread.

This is part of the go/surfce-flinger-scheduler move.

Test: All SF tests pass. Adding a Scheduler test in the next CL.

Change-Id: Ic45b662f82d60da1172bf64485c838344082b306