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
Change-Id: Ib262129422e6198f019482547a6b129cc60f0865
Merged-In: Ib262129422e6198f019482547a6b129cc60f0865
(cherry picked from commit f17b66e7)