SF: Intro CE::OutputLayer

Introduce compositionengine::OutputLayer, for representing the
composition state of a layer on an output (display)

This change just introduces the OutputLayer class, and modifies
SurfaceFlinger.cpp to create them properly. The new class does not yet
have any state.

Test: atest libsurfaceflinger_unittest libcompositionengine_test
Bug: 121291683
Change-Id: Ic8fe0fee61470b36539287883d4901b25208b634