Create subdirs in system_ce/ for multi-user backup

Backup system service bookkeeping is being moved to per-user CE
directories to support multiple users participating in the service.

Accompanies SELinux changes at aosp/873133

Bug: 121197420
Test: 1) Boot device; check dirs created with correct label; run backup
successfully on system user
2) Create secondary user; check dirs created with correct label; run
backup successfully

Change-Id: I3a0fdbfcf18a3c242fc64fba0dd014160b50b2f0
1 file changed