AVRCP: Stop MediaBrowser attempts on shutdown

Once we start our atttempts to connect to MediaBrowsers, we can get a
signal to shut down which means we could connect to a MediaBrowser and
then never disconnect, leaking a service connection.

Test: rapid on-off of the Bluetooth in settings
Change-Id: I0364b387f9b2e4ca1f65d122060ed4c153b9d70d
Fixes: 37634554