Added UserManager.isUsersOnSecondaryDisplaysSupported()

By default it reads the value from
config_multiuserUsersOnSecondaryDisplays
overridden by the system property for development purposes.

Test: adb shell setprop fw.users_on_secondary_displays true && \
      adb shell dumpsys user | grep -i "secondary display"

Fixes: 240613396

Change-Id: I1428a117a683e99fcaef71492be297c844d520e2