Remove observer when QSBH is detached

When QuickStatusBarHeader is detached (for example, when QSFragment is
destroyed) remove the LiveData observer.

Test: ahat, after dark mode (triggers re-inflate), there's only one QSBH
observer
Fixes: 155425341

Change-Id: Ic92367ea786228dfefb9808d9cad7d73197b4967