Fix bug #14288652 Settings crash on hitting back key from Tap and Pay screen

Gasp ... the Observer registering/unregistering code was not totally correct as
we were unregistering on a different RootAdapter.

- refactor the code for registering / unregistering the Observer
- unregister when the Fragment is stopped thru onStop().

Change-Id: I036eacd87c80fd2c9dedca705fb94a57a0c9a21d