Make UM.isUserVisible() and UM.getVisibleUsers() @SystemApis.

These APIs are "similar" to AM.getCurrentUser() which is also
@SystemApi. Besides, regular apps shouldn't care about users.

Test: m update-api
Test: atest CtsMultiUserTestCases:android.multiuser.cts.UserManagerTest MultipleUsersOnMultipleDisplaysTest

Fixes: 259423113

Change-Id: I71a5bcefdb719cee632936faf8bdf87386356452