add bgtask uid for SA downgrading

Signed-off-by: Nzhangxin_T <zhangxin312@huawei.com>

services/etc/passwd

@@ -65,6 +65,7 @@ work_scheduler:x:3047:3047:::/bin/false
 device_usage_stats:x:3048:3048:::/bin/false
 faceauth_host:x:3049:3049:::/bin/false
 pinauth_host:x:3050:3050:::/bin/false
+bgtask:x:3051:3051:::/bin/false
 dms:x:5522:5522:::/bin/false
 dbms:x:6000:6000:::/bin/false
 update:x:6666:6666:::/bin/false