[flexiglass] Dismiss keyguard when any scene goes to Gone.

We have a bug where moving to the Gone scene from scenes that are not
Bouncer (for example from lockscreen in Swipe) cause a black screen to
appear.

This happens because the code in KeyguardSecurityContainerController
which is responsible for dismissing the keyguard only did so for the
bouncer -> gone scene change.

The CL fixes the bug by allowing any change to the gone scene to trigger
the dismissal of the keyguard.

Fix: 295038434
Fix: 295223686
Test: unit tests still pass
Test: verified that I can see the launcher when swiping away the
lockscreen

Change-Id: Ied9fb452755a2791ee8bb28f0faa29f9d4ddfd0d
Merged-In: Ied9fb452755a2791ee8bb28f0faa29f9d4ddfd0d