Introduce getChildSurfacePackage() and clearChildSurfacePackage()

Add a way to determine whether there’s embedded content inside a SurfaceView with a SurfacePackage. setChildSurfacePackage absorbs RemoteException today, and we have no way to know whether the call succeeded. Adding such APIs improves testability.

Bug: 341021569
Test: SurfaceViewTest
Flag: android.view.flags.surface_view_get_surface_package
Change-Id: I60d33cecf0c3ecb7a7595baa0e2010e78c23faca