Initial implementation of UserManager.isUserVisible().

This API will be used to determine if the user is interacting with
the device and hence is able to do stuff like launching activities.

This initial version only returns true for the current foreground
user, but it will be expanded to also return true for:

- Background users associated with secondary displays
- (Running) profile of other visible users

Test: atest CtsMultiUserTestCases:android.multiuser.cts.UserManagerTest#testIsUserVisible_currentUser
Test: manual verification with Automotive's KitchenSink
Test: m update-api
Bug: 239824814

Change-Id: Ie8da07f69898d2b376bcf60d1da4221aed03ee3b