Avoid creating futures for drawables with no constant state

We don't need to create futures for drawables without constant state,
since we only copy on mutate and we don't need to do any work on mutate()
for drawables without shared constant state. Also we would crash in that
case, so avoiding the NPE is nice too.

Rider: Also fixes elevations again.

BUG: 18696100
Change-Id: I4d7737f39ce3efc5830704e5ce412c540603e6ac