CE: Add test coverage for Output::composeSurfaces

As part of adding coverage, `PrintTo` and `operator==` were defined for
both RenderEngine::DisplaySettings and RenderEngine::LayerSettings so
that Google Test could use them.

Bug: 144115950
Test: atest libcompositionengine_test
Change-Id: Ia22c822f705e890435e81c0b75b4986afde49317