Use consistent method to get current user

Depending on the timing of the switching actions, the
KeyguardUpdateMonitor and the user tracker may have different current
users. We should use a consistent method for getting the current user in
the KeyguardViewMediator

Bug: 250675259
Test: manual
Change-Id: I2b078ac21f0546d1b8de24f55e4225a754d53aea