Removing accessibility gesture from power dialog.
Adding new accessibility shortcut activated by holding both volume
buttons down. This shortcut is configurable by OEMs and users to
work with any installed accessibility service.
Bug: 30160335
Test: Added automated testing for the EnableAccessibilityController.
Manually toggled various services on and off.
Change-Id: I546bd29a2ab1ba64a0cbfd11e2004cdf85ee6cfd
106fe73205