SF: Refactor onLayerDisplayed

Adds a new LayerFE::onLayerDisplayed interface, and adjusts the
front-end Layer to match.

Adjusts the code in SurfaceFlinger to use the new interface, in
preparation to moving the loop over to CompositionEngine.

Test: atest libsurfaceflinger_unittest libcompositionengine_test
Bug: 121291683
Change-Id: I9231f4b425ea1b9f37363521c7e3f6aef63bcec6