Touch-exploration improvements to volume dialog.

 - Extend the dismiss timeout when interacting with various
   subcontrols.
 - Ensure "hover" events in touch exploration extend the timeout
   in addition to touch events.
 - Introduce new helper to standardize interaction callbacks.
 - Announce zen toasts.
 - Announce zen condition selections, and when existing countdown
   conditions are modified.

Bug:17578434
Change-Id: I8a055b3455aa8d20ba93439bdec6cc75db97800e