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

vold_prepare_subdirs.cpp

@@ -140,6 +140,15 @@ static bool prepare_subdirs(const std::string& volume_uuid, int user_id, int fla
             if (!prepare_dir(sehandle, 0700, 0, 0, misc_ce_path + "/vold")) return false;
             if (!prepare_dir(sehandle, 0700, 0, 0, misc_ce_path + "/storaged")) return false;
             if (!prepare_dir(sehandle, 0700, 0, 0, misc_ce_path + "/rollback")) return false;
+
+            auto system_ce_path = android::vold::BuildDataSystemCePath(user_id);
+            if (!prepare_dir(sehandle, 0700, AID_SYSTEM, AID_SYSTEM, system_ce_path + "/backup")) {
+                return false;
+            }
+            if (!prepare_dir(sehandle, 0700, AID_SYSTEM, AID_SYSTEM,
+                             system_ce_path + "/backup_stage")) {
+                return false;
+            }
         }
     }
     return true;