Improvements on "start bg user on secondary display":

- Adds hidden API on ActivityManager
- Checks if feature is supported
- Adds shell command to check if user is visible
- Includes (visible) on 'cmd user list -v'

Test: atest CtsMultiUserTestCases:android.multiuser.cts.UserManagerTest#testIsUserVisible_backgroundUserOnSecondaryDisplay
Test: m update-api
Test: adb shell cmd user is-user-visible --display 0 cur

Bug: 239982558

Change-Id: Ie7fe8c832ef2a3ffe4869a67fd35a0da363d0039