Treat device storage as a single preference.

Make all running instances recognize changes to pref and update
    ui live...eliminates stale UI for this pref.
Also (of not to suche@) we're now starting and stopping the
    monitor in the onCreate and onDestroy methods. Can
    you advise if there was a reason we were previously
    doing so in pause and resume?
Update TestPredicate to have a waitForCall method (backed by a Future).

Bug: 35993931
Test: Build and test.
Change-Id: I59437f163e5cf928bb8131374c098a6579f9eb81