Simplify and consolidate dismissAlpha

This fixes alpha whencanceling a swipe to unlock action; the user
drags up to dismiss but then drags back down to go back to the
lockscreen. Move all logic to the KeyguardInteractor, and simplify
SharedNotificationContainerViewModel alpha.

Test: atest KeyguardInteractorTest
Fixes: 345738121
Flag: com.android.systemui.migrate_clocks_to_blueprint
Change-Id: I65ebee06349c302233928fc6564085787378adcc