Fix NPE in UiModeManagerServiceTest

Replace enforceValidCallingUser() in UiModeMaangerService with
UserManagerService#enforceCurrentUserIfVisibleBackgroundEnabled().
This fixes the NPE in UiModeManagerServiceTest that was caused by
UserManagerService not being instantiated in unit tests.
Also remove redundant UiModeManagerService#enforceValidCallingUser
method.

Bug: 360925919
Flag: EXEMPT bugfix
Test: atest FrameworksUiServicesTests:com.android.server.UiModeManagerServiceTest

Change-Id: I6c6b23c55835c535dd8d140b21332a8f141eeba1