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 f17b66e731224605373efc27c448a048e954d663)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:12238e8fc6e8bfc2c4ae6100fefb0cf99ec17a76
Merged-In: Ib262129422e6198f019482547a6b129cc60f0865
Change-Id: Ib262129422e6198f019482547a6b129cc60f0865