[flexiglass] Invoke IKeyguardDismissCallbacks

When we get past the bouncer by unlocking the device or when we leave
the bouncer without unlocking, these callbacks need to be invoked.

This helps, for example, the occluding camera app know to switch to its
insecure version where the user can see the gallery button.

Fix: 348646872
Test: manually verified that tapping on the lock icon in the occluding
camera activity's gallery button (bottom left), unlocking via bouncer or
via face unlock, correctly returns to the camera app, but this time it's
unlocked.
Test: manually verified that exiting the bouncer instead invokes the
onDismissCancelled method on the callbacks
Flag: com.android.systemui.scene_container

Change-Id: I95d9f2205dd022fca6ee2b5d8a2e339d379c4acf