Ensure exit animations are canceled prior to user switch

On user switch, remove any pending requests and set a flag to indicate
a cancelation, and verify that flag just prior to handling the
exit animation callback from window manager.

Bug: 407562568
Test: atest KeyguardViewMediatorTest
Flag: EXEMPT bugfix
(cherry picked from commit f17b66e7)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:59a4e4f5fcdb40220a208519a8c992839a8d91e6
Merged-In: Ib262129422e6198f019482547a6b129cc60f0865
Change-Id: Ib262129422e6198f019482547a6b129cc60f0865