Cache Scene objects by layout ID on the scene root

The current tracking of scene objects in a static ThreadLocal is
problematic as it leaks the Context associated with the SceneRoot and
returns the wrong Scene object if the same layout ID is used across
different scene roots.

Track Scene objects on the scene root view instead to avoid these
issues.

Change-Id: I891986897f757f2666897c937b5ebb0ed1d531c1