Quick optimization for committing Layers

Layer::commitChildList is a traversal by itself. Running it inside a
traversal results in an undesired n^2 traversal time.
commitChildList is swapping the common state with the drawing state and executes the assignment multiple times / Layer, which could become quite costly.


Change-Id: I15f670b7766a3c2a16abbd3ae20bb8a7c76ec852
Test: atest SurfaceFlinger_test - all tests pass.