This allows an AccessibilityService to set a flag in its
AccessibilityServiceInfo that triggers the navigation bar to show an
Accessibility Button and observe callbacks when the button is clicked
or there are changes in the visibility of the navigation bar.
Test: Manual (Created a sample AccessibilityService) + CTS
Bug:29231271
Change-Id: I03d653d85bc37df28ed71d8bba94b7c75fe56e43