More minor improvements on users on secondary display APIs.

- Removed warning on UserManagerService.getUserAssignedToDisplay().
- Improved javadocs on UserManagerInternal and ActivityManager.
- Explicitly throws a UnsupportedOperationException on
  ActivityManager.startUserInBackgroundOnSecondaryDisplay() when it's
  not supported (and add a CTS test for it)
- Don't allow user to be assigned to multiple displays.

Bug: 244644281
Test: atest CtsMultiUserTestCases:android.multiuser.cts.UserManagerTest#testStartUserInBackgroundOnSecondaryDisplay \
            MultipleUsersOnMultipleDisplaysTest:android.multiuser.cts.UserManagerTest#testStartUserInBackgroundOnSecondaryDisplay

Change-Id: I01375d70b746148f99c4a5ff1eea6faf83154a86