Avoid overwriting SYSTEM_ADJ with PERSISTENT_PROC_ADJ

If an application running on system_process has a persistent flag,
the oom_adj value for system_process is unexpectedly overwritten
to PERSISTENT_PROC_ADJ (-12).

We must keep SYSTEM_ADJ (-16) for system_process.
This change guards against overwriting SYSTEM_ADJ with
PERSISTENT_PROC_ADJ

Change-Id: I8f334d93b82271f9a0cfbbef34a75af2dacb186b