Changes AM.startUserInBgOnSecondaryDisplay() to check if display is valid.

Also adds APIs to list the valid displays:
- @TestApi AM getSecondaryDisplayIdsForStartingBackgroundUsers()
- cmd activity list-secondary-displays-for-starting-users
- system property
  (fw.secondary_display_for_starting_users_for_testing_purposes)
  to override it for testing purposes

Test: m -j update-api
Test: adb shell cmd activity list-secondary-displays-for-starting-users
Test: atest MultipleUsersOnMultipleDisplaysTest # CTS
Test: atest ActivityManagerServiceTest ActivityManagerServiceInjectorTest # unit

Bug: 239982558

Change-Id: Iaf5c83344075cce9985885a3f9359ebbd0174d41