Exclude a private display from getDisplayIdsForStartingVisibleBackgroundUsers

A private display (e.g. cluster dipslay in Automotive) may not be used
for starting a visible background user, so should be exlcuded from the
results of
ActivityManagerService#getDisplayIdsForStartingVisibleBackgroundUsers.

Bug: 284489759

Test: adb shell cmd activity list-displays-for-starting-users
      atest ActivityManagerServiceTest

Change-Id: Ic34c10cf8736e6870e4a8b9be2eb17ce280a44b3