Integrate keyguard behavior in split screen with shell-transition

Support to hide the divider bar when keyguard showing and support to
dismiss split after unlocked on folded device. Also unbind
show-when-locked split dismissing flow with stage visibility changed
callback to simplify the code.

Bug: 206487881
Test: atest WMShellUnitTests
Test: trigger split, won't see divider bar showing above keyguard.
Test: trigger split, fold the device and unlock will dismiss split. And
      there's no such split pair in recents.
Test: trigger split with show-when-locked activity, it'll dismiss split
      when it's occluding keyguard.
Change-Id: If536121ef0cae4acda12268ffb32024a9736bacd