Add support for dismissing in Talkback Mode

This should let the user be able to access dismiss in talkback
mode. However, because talkback relies on focus changes to work,
the focus change animator is disabled. This means some visuals
will not work the same as when talkback is turned off.

BUG: 28594452
Change-Id: Idb443fe559bd7a636ff381e6727f81d41d0bd325