Accessibility enabled by gesture not reflected in settings screen.

1. We have added a global gesture to enable accessibility which is long press
   on power followed by a long press with two fingers. If this gesture is
   performed when the settings for an accessibility service 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: I214c64877086eeb1fe156bcc195aa4e2931d6bdb