Added system_server to system group

Test: on gobo
pidof system_server appears in the correct file:
$ adb shell pidof system_server
859
$ adb shell cat /dev/memcg/system/tasks | grep '859'
859

Bug: 65130832
Change-Id: Id63abd473a7f2001688b0be06bfec6472c7f8648
1 file changed