init.rc: use oom_score_adj instead of oom_adj

Use kernel oom_score_adj interface to make init and children unkillable.

Stop using older, deprecated oom_adj interface.

Use OOM_SCORE_ADJ_MIN to make the processes unkillable (previously the processes
were set to a very low score, but not unkillable).

Change-Id: I680965009585c2a5a580859fb946f2d0caa95d9c