Accessibility settings should update after gesture to enable accessibility.

1. We have added a global gesture to enable accessibility which is long press
   on power follower by a long press with two fingers. If this gesture is
   performed when accessibility settings are on the screen, accessibility and
   the screen-reader are enabled while the UI does not reflect that and the
   user may try to enable the already enabled screen-reader which will show
   two irrelevant warning dialogs - one for enabling the screen-reader and
   one for enabling explore by touch (assuming the screen-reader requests it).
   This change adds a simple content observer for the relevant accessibility
   settings which upon a change calls the functions to refresh the settings UI.

bug:7309127

Change-Id: I45851d7789e24e9e0a5bc5aaac39db9a12b60e0c