Allow backup payload for shortcuts from secondary user

Bug: 260066878
Test: verified using following steps
1. Add a secondary user in Settings -> System -> Multiple users.
2. Set up the secondary user and sign in with a GAIA account.
3. Pin some shortcuts to the workspace.
4. Run adb shell bmgr --user {your secondary user's ID e.g. 10} activate
   true
5. Under Settings -> System -> Backup, opt into backups and press "Back
   up now" -- this can take a few minutes.
6. adb pull /data/system_ce/0/shortcut_service/shortcut_dump/
backup-1-payload.txt Verify the backup payload contains pinned shortcuts.
7. Factory reset and perform a cloud restore, verify pinned
   shortcuts are restored successfully.

Change-Id: I0ce48aff337085476087f0db882f3a117416eea6