Mock Resources.getSystem in UserManagerServiceTest

UserManagerService uses Resources.getSystem() to get Resources, along
with other methods.
This CL makes UserManagerServiceTest mock Resources.getSystem, so it is
possible to test with different resource values there, in future CLs.

This CL also moves shouldShowNotificationForBackgroundUserSounds(), so
that we do not have one service class call another.

Bug: 390496349
Flag: EXEMPT refactor
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest
Test: atest FrameworksVibratorServicesTests:com.android.server.vibrator.VibratorManagerServiceTest
Change-Id: Ieb730dd9f3d3c36d1bdf2ff555871f2539971030