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