[flexiglass] Don't switch to gone if already switching to LS.

Fix: 330476229
Test: unit test added
Test: manually verified by turning off the display and turning on the
display, with pattern as the auth method, with "lock instantly" turned
on or off and with AOD turned on or off; compared the behaviour to
pre-Flexilgass and vierifed it's the same.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I1c580b07ddb6d42c3cd901910cea3df6c4f36c29