Renamed methods related to starting user in bg visible on display.

The existing methods assumed the user could only start in background
on secondary displays, but this assumption might change (for example,
in cars with 2 Android systems, one just for back-seat passengers).

Test: atest ActivityManagerServiceInjectorTest ActivityManagerServiceTest UserControllerTest UserVisibilityMediatorMUMDTest UserVisibilityMediatorSUSDTest # unit
Test: atest UserVisibilityMultipleDisplaysTest UserVisibilityTest # CTS
Test: m update-api && m
Test: adb shell am list-displays-for-starting-users
Test: adb shell dumpsys user --visibility-mediator

Fixes: 261538216

Change-Id: I8b7b30fb48d9098cedaac243560693e6c0e20ed7