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

(cherry picked from commit 45d8174f)

Change-Id: I68c18f9db24d55239f7f0608592fcc702f04542e