[DO NOT MERGE] Keyguard - Treat messsages to lock with priority

When switching users and attempting to lock the device, the sysui main
thread becomes overwhelmed with events, creating a significant lag
between the time a message is posted and processed on the main
thread. This can be dangerous when these events are critical for
security, such as calls coming from PhoneWindowManager#lockNow() that
call KeyguardViewMediator#doKeyguardTimeout(). On older devices with
slower CPUs and less memory, the delay in processing can be
significant (15 - 30s).

The result of not prioritizing these events leads to a window of time
where a guest user can switch back to the owner, and gain access to
the owner's homescreen without needing to unlock the device with the
owner's credentials.

As a mitigation, prioritize two events originating in two specific
methods to make sure the device locks as soon as possible as well as
have the system server preemptively update its local cache.

Bug: 151095871
Test: Very manual race condition - follow steps listed in bug
Change-Id: I7585a0a5eeb308e0e32a4f77f581556d883b5cda
Merged-In: I7585a0a5eeb308e0e32a4f77f581556d883b5cda
(cherry picked from commit 28c53ab8)