Add EmptyActivity to SceneCore integration testapp

Adds a blank EmptyActivity host activity and registers it in the
SceneCore integration testapp manifest to support isolated,
hermetic instrumented integration tests via ActivityScenario.

Test: ./gradlew :xr:scenecore:integration-tests:testapp:compileDebugAndroidTestSources
Change-Id: I149c92760b6ea819e85bfc5193097777471f1d93