Fix KeyguardUpdateMonitor test

KeyguardUpdateMonitor's Handler runs on the main looper by design,
we need to ensure that whenever we're processing messages, that
they will also be received on the main looper.

Also unregistered a broadcast listener to avoid a possible race
condition

Test: atest packages/SystemUI/tests/src/com/android/keyguard/KeyguardUpdateMonitorTest.java
Bug: 79550837
Change-Id: I14a319da5c6bc46fd32675ae205e14a6228efaa4