CE: Layers only have FE layer state

Remove the concept of having a compositionengine::Layer having internal
state since it was only used to internally store a copy of the
LayerFECompositionState. Instead switch the Layer interface so only the
FE state can be obtained. Any internal state will from now on be
private.

Doing this also allows the LayerFECompostionState to be easily extended
by extended versions of CompositionEngine.

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: I689e8714aca46320c87e0cb5ef18fdde93eb7499