SF: Move/Refactor rebuildLayerStacks and computeVisibleRegions

This is the one big final move of code to CompositionEngine.

The logic is restructured, though the basic blocks still correspond to
what existed before to allow for easier conflict resolution across the
move.

The function to compute the per-layer visibility information is still
much bigger than I like -- perhaps this can be cleaned up later after
more unit test coverage is introduced.

Test: atest libsurfaceflinger_unittest libcompositionengine_test
Test: atest CtsColorModeTestCases
Test: atest CtsDisplayTestCases
Test: atest CtsGraphicsTestCases
Test: atest CtsUiRenderingTestCases
Test: atest CtsViewTestCases
Test: atest android.media.cts.EncodeVirtualDisplayWithCompositionTest
Test: go/wm-smoke
Bug: 121291683
Change-Id: Ia3fb6c0fb0bb847ab737a1f92617d3724c08ea54