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
diff --git a/services/surfaceflinger/CompositionEngine/src/CompositionEngine.cpp b/services/surfaceflinger/CompositionEngine/src/CompositionEngine.cpp
index 855507e..a4e1fff 100644
--- a/services/surfaceflinger/CompositionEngine/src/CompositionEngine.cpp
+++ b/services/surfaceflinger/CompositionEngine/src/CompositionEngine.cpp
@@ -140,6 +140,10 @@
     mNeedsAnotherUpdate = needsAnotherUpdate;
 }
 
+FeatureFlags CompositionEngine::getFeatureFlags() const {
+    return {};
+}
+
 void CompositionEngine::dump(std::string&) const {
     // The base class has no state to dump, but derived classes might.
 }