When wallpaper target is under keyguard, use keyguard

The case this is specifically aimed at is recent apps over the
lockscreen, where the wallpaper target isn't visible during the
transition or at its end but is replaced by NotificationShade when
it reports finished.

The problem this solves is that the wallpaper visibility cleanup in
Transition.onFinish was run before the NotificationShade had a chance
to make itself wallpaper target, as this was not necessarily
recalculated yet.

For normal app <-> app transitions, wallpaper target is calculated much
earlier during collection. Since keyguard is just a mode of the
notification shade there is no collection to be done for it and so the
wallpaper target change is not knowable upfront.

We don't use the wallpaper as its own target in this case because the
keyguard does not necessarily want a wallpaper - it may become an opaque
window on some form factors / skins.

Bug: 315698413
Test: atest WallpaperControllerTests
Test: atest PlatformScenarioTests:android.platform.test.scenario.sysui.lockscreen.LaunchCameraFromLockScreen
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:719144b3a596a9ca0b7161dfe972eb8843939732)
Merged-In: Iaa631b75d5bc24444bd7c808e0aa056620edcc4b
Change-Id: Iaa631b75d5bc24444bd7c808e0aa056620edcc4b