Revert "Omit DISPLAY_DECORATION layers from CachedSets"

This reverts I1a415f9b21446dbbe60488d66b00a5da231d42c3.

Because the A8 layer is uncompressed, it saves power to merge
DISPLAY_DECORATION layers into a CachedSet, even if the CachedSet would
not otherwise be fullscreen.

Rewrite the test to verify the new behavior.

Bug: 225187874
Test: power tests
Test: libcompositionengine_test
Change-Id: I7b3fe17d61fed100a78d22175117fdc32c890aba