init: set oom_adj early before fork vendor_init

right now vendor_init is forked before we set oom_adj for init which
leaves a chance vendor_init could be killed in heavy memory pressure.

this CL set the oom_adj before forking everything to ensure all native
have correct oom_adj settings.

Fixes: 130824864
Test: procrank -o
Change-Id: I8af129076c3efa29f7b781459449f8f2dc853c98