Remove all pending keyguard exits on SHOW

Noticed during rapid requests to enabled/disable keyguard,
prevent any preexisting requests to hide or dismiss keyguard
from running when a SHOW message is handled. Reset goingAway,
and also make sure to prevent any WM callbacks in flight from
continuing.

Test: atest KeyguardViewMediatorTest
Bug: 395640609
Bug: 419095873
Flag: EXEMPT bugfix
(cherry picked from commit 8853b2c7)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:31d93f9896ee0fde5f3ee82346c2a9d2f2069cd3
Merged-In: I732fb7d577a5f4c65c8d84cc66a61f5006ed8d53
Change-Id: I732fb7d577a5f4c65c8d84cc66a61f5006ed8d53