CE: Introduce Output::GetDisplayId()

Allow anyone with an Output interface pointer to get the displayId (if
any) associated with the output.

As the return value is optional, an output that does not have a
displayId can return std::nullopt

Test: atest libsurfaceflinger_unittest libcompositionengine_test
Test: go/wm-smoke
Bug: 121291683
Change-Id: Ie34da86aa4b55d11a291156e85dae678d7df5ee2
diff --git a/services/surfaceflinger/CompositionEngine/src/Display.cpp b/services/surfaceflinger/CompositionEngine/src/Display.cpp
index 405ad42..e885629 100644
--- a/services/surfaceflinger/CompositionEngine/src/Display.cpp
+++ b/services/surfaceflinger/CompositionEngine/src/Display.cpp
@@ -55,6 +55,10 @@
     return mIsVirtual;
 }
 
+std::optional<DisplayId> Display::getDisplayId() const {
+    return mId;
+}
+
 void Display::disconnect() {
     if (!mId) {
         return;