Refactor getCurrentUser out of KeyguardViewMediator

KeyguardUpdateMonitor.getCurrentUser() is now removed after all
references were refactored. setCurrentUser() is not used therefore and
just `notifyTrustedChangedLocked()` had to be moved to `onUserSwitching`
 instead. Ideally these calls would listen to a flow of
 getSelectedUserId() such that any change to the user would be reacted
 to instead of relying on `onUserSwitching` callbacks.

Test: Refactored tests
Bug: b/303808405
Bug: b/297839820
Flag: b/305984787 - REFACTOR_GETCURRENTUSER (added in ag/25089323)
Change-Id: I7b1ca0d30f56ad512a5e454e738e8c07ac906e38