Add keyguard-appearing transition handler

This lets the SysUI implementation combine the keyguard appearing and
occluding events into a single transition after it has called
setLockScreenShown, so that it has all the information needed to play
the right animation in one step.

At the moment we won't play any animation and always just jump cut.

Flag: com.android.window.flags:keyguard_appear_transition
Bug: 337917376
Bug: 338606332
Bug: 338008315
Test: KeyguardManagerTest
Test: Crystalball run b/338008315#comment7
Change-Id: Id8e8518614386f8a3d92d4587701a96deb77fcef