Implements the aod appearing transition

This is to show lock wallpaper according to transition
lifecycle. WmCore requests the appearing transition when
aod is showing. And then, transition collects all wallpapers
including system and lock. KeyguardService can apply
cross-fade animation for wallpapers.

Bug: 361438779
Test: Manual
      Setup lock wallpaper
      Turn screen off to go aod state
Flag: com.android.window.flags.aod_transition
(cherry picked from https://partner-android-review.googlesource.com/q/commit:0f4f0a2689a545f2b4cf3f0979b1a89b15162b0d)
Merged-In: I8b9bae3d39124ee6f983975023b9d1d98b3700c6
Change-Id: I8b9bae3d39124ee6f983975023b9d1d98b3700c6