CE: Snapshot layer metadata

Adds LayerMetadata to LayerSnapshots. LayerMetadata is only populated
when CE enables the new feature flag and the feature flag will only be
enabled in ARC++'s CE.

This change is necessary to split LayerFE from the Layer class as
ARC++'s LayerFE subclass accesses LayerMetadata.

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

Change-Id: Iae405707897eedf5af9025fe628c1cd60b596e10