New accessibility shortcut.

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