mm/oom_kill.c (affects lowmemorykiller): prevent changing oom_score_adj if its value is -1000

This should prevent that the Android OS automatically changes
its value if oom_adj -17 or oom_score_adj -1000 were explicitly
set by the user. To gain control over the variable again,
oom_adj has to be set to anything higher than -17.

Also: compare p->signal->oom_score_adj with OOM_SCORE_ADJ_MIN instead
of static value.

Signed-off-by: Dan Trifan <jimsth@gmail.com>