SF: Delegate ICompositor interfacing to Scheduler

Factor out MessageQueue's calls to commit/composite/sample into an
onFrameSignal delegate that Scheduler implements, since the policy
for ICompositor interfacing (e.g. whether to composite displays
sequentially or concurrently) will be the latter's responsibility.

Improve type correctness by replacing more nsecs_t with TimePoint/
Duration, and int64_t with a new VsyncId type.

Bug: 241285191
Test: Boot
Change-Id: I5039894038b62a562c1f93b7cea41751f7f76ca6