Allow visible background users on default display.

This scenario will be used on automotive builds that only support
passengers (for example, when a 2nd instance of Android runs on the
back seat of a car).

Test: atest UserVisibilityMediatorMUPANDTest UserVisibilityMediatorMUMDTest UserVisibilityMediatorSUSDTest # unit
Test: atest UserVisibilityVisibleBackgroundUsersOnDefaultDisplayTest # CTS
Test: m update-api
Test: adb shell stop && \
  adb shell setprop fw.visible_bg_users true && \
  adb shell setprop fw.visible_bg_users_on_default_display true && \
  adb shell start && sleep 30 &&
  adb shell cmd user is-visible-background-users-on-default-display-supported &&
  adb shell am start-user --display 0 10

Fixes: 261538232

Change-Id: I043b0677351999182a73ed938b2da38c955d5433