logcat: logcatd: make logcatd killable

Near last resort kill logcatd, set /proc/self/oom_score_adj to -600.

Can kill to keep system_server, telephony and bluetooth.

Test: manual, start logpersist, runs
Bug: 62147352
Change-Id: I3723761bf1d2b79ce6a9557d2e78f91cc5497e96