Add setOnDismissListener to AlertDialog.Builder

Add documentation to setOnCancelListener to clarify that the cancel
event plus the events for the various choice buttons are not the
exhaustive set of ways the dialog can be dismissed, and that a dismiss
listener should be used if the app needs to cover all cases of
dismissal.

Change-Id: I9e9d6f90f6f9ccaeb2c697474ab353e2d78f37b9