Keyguard: Defer doze-to-lockscreen transition on shade drag

- When the device transitions from dozing to the lockscreen at the same
  time the user pulls down the notification shade, their respective
  alpha animations can conflict, causing visual glitches.
- This change introduces logic to prioritize the shade drag animation.
  A new flow, dozingToLockscreenAlpha, monitors the shade expansion
  alpha. If the shade is being dragged, the doze-to-lockscreen alpha
  animation is suppressed by switching to an emptyFlow().

Bug: 419763981
Flag: com.android.systemui.defer_doze_transition_on_shade_drag
Test: atest KeyguardRootViewModelTest
Test: Manual - b/419763981#comment10
Change-Id: Ie037c88b7762a4d33a76380c558c42a2d48c8b12