SF: Decouple MessageQueue

Define an ICompositor interface against which MessageQueue (which ought
to be an implementation detail of Scheduler) is implemented. Change the
equivocal invalidate/refresh nomenclature to commit/composite. Schedule
sampling only after composite.

Bug: 185535769
Test: libsurfaceflinger_unittest
Change-Id: I0c18f312459bae48531449f24f7b53c104fc5812