SF: Split FE and CE

Make LayerFE a standalone class with no links back to Layer. Pass
LayerSnapshot from Layer into LayerFE before CompositionEngine::present
and back after.

Bug: 238781169
Test: go/wm-smoke
Test: presubmit

Change-Id: I5395fb717a931f88e2bf26395acd21e8b308961e
19 files changed