Refactor isVisibleBackgroundUser into UserManagerService

A visible background user is a full, background user assigned to
secondary displays on the devices that have
config_multiuserVisibleBackgroundUsers enabled. The logic for checking
visible background users was used repeatedly in a couple of classes,
so the logic was moved to UserManagerService to remove redundant codes.

Bug: 342315883
Flag: EXEMPT refactor
Test: m

Signed-off-by: Dohyun Lee <dohyun.lee@lge.com>
(cherry picked from https://partner-android-review.googlesource.com/q/commit:7e2440877925659abbd21d8bc8f213fc4db58f45)

Change-Id: I527bb05f690c6a6c9c98fbd8753890030e8536a3