Invoke dismiss callback from alternate bouncer

On back gestures, the primary bouncer will not show. The
primary bouncer notifies the dismiss callback to cancel,
so do the same for this code path.

Also, add logging to the dismiss callback.

Fixes: 353864564
Test: atest AlternateBouncerToLockscreenTransitionViewModelTest
Test: manual, follow steps in bug
Flag: EXEMPT bugfix
Change-Id: I40d8fcc4e7c92e23ecf9fb6c0dc9be42347c3ae6