Fix bug #16957601 Stability: ISE in Settings: Observer com.android.settings.

SettingsPreferenceFragment$1@273c8fdb was not registered

- add override for onUnbindPreferences() that will unregister the observer
if needed
- keep track of the root adapter so that we can unregister / register with
the correct one (by having a new PreferenceScreen the root adapter would not
be the same and thus unregistering the Observer on the new one would not work
and create the current bug)

Change-Id: I2cef0398c2ae0ab4f5ffd67ca20e8874be997bf6