Initial implementation of "start bg user on secondary display".

This API will be used on automotives that support multiple displays
to start a passenger user on secondary displays. Once a user is
started this way, it will be "visible"
(i.e. UserManager.isUserVisible() will return true).

This CL provides the very minimum support to such API
(A.K.A. "My Dog, it's full of TODOs!" :-), so it can unblock other
efforts (like changing WindowManager to allow activities to be
launched).

Bug: 239982558

Test: adb shell am start-user --display 42 10
Test: adb shell dumpsys user | egrep '(Users on secondary|isUserVisible)'

Change-Id: I21dc85720da7d0adc24ca9f4573b85ef5a380239