Update edgeBackGestureHandler for Taskbar on init/destroy()

* TaskbarDelegate wasn't calling
mEdgeBackGestureHandler.onNavBarDetatched() when going from
large screen to phone screen
* Also cache taskbar visibility to prevent multiple instances
of the same listener from being added

Bug: 197058461
Test: Went from tablet to phone, used back,
saw Launcher#onBackPressed() only got called once

Change-Id: I63b742b047cbfac1421c5a0e8422e9e341c62627