Merge metadata from transaction instead of replace

This was replacing metadata when set in transactions rather
than merging. To fix this, merge has been augmented to also
report if a change occurred and to erase empty entries (as
a mechanism to "unset" metadata)

Bug: 122925737
Test: Added more unittests
Change-Id: Ia854cbcc1ddd334f18ffacea667cbb98778ec210
diff --git a/services/surfaceflinger/Layer.cpp b/services/surfaceflinger/Layer.cpp
index 2de169d..646402c 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -1334,10 +1334,8 @@
     return true;
 }
 
-bool Layer::setMetadata(LayerMetadata data) {
-    bool changed = data.mMap != mCurrentState.metadata.mMap;
-    if (!changed) return false;
-    mCurrentState.metadata = std::move(data);
+bool Layer::setMetadata(const LayerMetadata& data) {
+    if (!mCurrentState.metadata.merge(data, true /* eraseEmpty */)) return false;
     mCurrentState.sequence++;
     mCurrentState.modified = true;
     setTransactionFlags(eTransactionNeeded);