Check status of mEdgeBackGestureHandler to prevent NPE

Sometimes onAttachedToWindow() can be called before
mEdgeBackGestureHandler is created, which led to a NPE. This change adds
a null check to prevent the crash.

Fix: 303820749
Test: Set mEdgeBackGestureHandler to null and ensured no NPE occured in
onAttachedWindow()

Change-Id: I43ab8bcfc4d720bc537a7ca36e08068a6a27fbea