Add system_server to system group only if per_app_memcg is true

P18 or other high-performance devices shouldn't use per-memcg so
it was disabled on those devices[1]. However, we didn't turn off
for system_server memcg so those devices still have two seperate
LRU groups which would have less efficient memory reclaim.

Therefore, this patch makes memcg use for system_server only if
per_app_memcg is enabled.

[1] https://googleplex-android-review.git.corp.google.com/c/platform/system/core/+/4178592



Bug: 110858093
Test: confirm via cat /dev/memcg/system/tasks is none
Change-Id: Ic0e3157bf8ff5de69767797b5508946ca3efeb68
Signed-off-by: Minchan Kim <minchan@google.com>