lmkd: Change error to warning when accessing oom_score_adj file

Occasionally processes are killed or crash while lmkd is trying to access
their oom_score_adj file. This is not necessarily an erroneous condition
and therefore should not be reported as an error. Demoting error to a
warning with a detailed message.

Bug: 72039129
Test: verified logcat output
Change-Id: I97444ba5198c02cb7f7ba03c3af12e4aad9d233e