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
Change-Id: I732fb7d577a5f4c65c8d84cc66a61f5006ed8d53