Refactored user start mode boolean into a tri-state int.

Prior to Android U, a user would start either on foreground (A.K.A.
"current user") or background (in which case it could not launch
activities). But now, the background user could be associated with a
display, in which case it would become "visible" (and could launch
activities in that display).

So, this CL changes the foreground / background boolean state into a
new tri-state int (@UserStartMode):

- USER_START_MODE_FOREGROUND
- USER_START_MODE_BACKGROUND
- USER_START_MODE_BACKGROUND_VISIBLE

But it doesn't change any logic (yet :-).

Test: atest FrameworksServicesTests:UserControllerTest
Bug: 261537398

Change-Id: I30fd3b0f4ea3fa03c3520214b360fc6920138f9f