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: com.android.systemui.keyguard_show_cancels_all_pending_exits
Change-Id: I0c70f09c5b736b108eb6cef9249dc5c90a7fd4cd