Refactor out KeyguardUpdateMonitor.getCurrentUser() 2/X

This commit refactors static references from KeyguardUpdateMonitor
.getCurrentUser() to UserInteractor.getSelectedUserId() to consolidate
one single source of truth in the entire code base.

This CL submits a bulk of these refactors that are "easy" to do. These
changes are not supposed to contain any changes in logic. There will be
subsequent smaller CLs for cases where injecting UserInteractor creates
more complicated to resolve dependency cycles.

Test: Ran all Sysui tests
Bug: b/303808405
Bug: b/297839820
Flag: b/305984787 - REFACTOR_GETCURRENTUSER (added in ag/25089323)
Change-Id: I9deb87b90db823bc2e91e7ceb961d9c23dc4da18