Cache accessibility info in NavBarHelper to reduce binder calls

- Currently, we make two binder calls whenever the sysui state changes
  to fetch the current accessibility info.  Instead we can use the
  AccessibilityButtonTargetsObserver to listen for the changes and
  only update the accessibility state when it changes.

Bug: 201633154
Test: Enable/disable accessibility services and verify button shows
      Enable/disable > 1 accessibility services and verify that
      button is long-clickable to show the services menu
      Verify both of the above with gesture nav & accessibility service
      as well

Change-Id: I84a7d20040c61ea75ac0b9e1f790f4a8b0ebe398