lmkd: Suppress error when accessing soft_limit_in_bytes for system_server

system_server needs to register with lmkd, however it has no memory
cgroup under /dev/memcg/apps. This change detects if the process being
registered is system_server and suppresses the error message when
/dev/memcg/apps/uid_%d/pid_%d/memory.soft_limit_in_bytes file can't
be accessed.

Bug: 73483785
Test: verified logcat output
Change-Id: I03df7831f41f512ac8d3ebc46330546d08a3cbc6