Turn on multi-user support for PreferredActivityBackupHelper

PreferredActivityBackupHelper is already multi-user aware. Adding PREFERRED_HELPER to sEligibleForMultiUser array should enable backups
for secondary users.

Bug: 261720100
Test: create secondary user
  adb shell bmgr --user 10 activate true
  adb shell bmgr --user 10 enable true
  create backup
  test restore
Change-Id: I9d5d37a06dad6ddc376cb8ec55af3e3d1afa0230