give zygote AID_READPROC

In zygote wrapping mode, ZygoteConnection does a check to see if the pid
reported by the wrapped process is either child process that was
forked, or a decendent of it. This requires read access to other
processes /proc files. Grant zygote AID_READPROC to allow this access.

Bug: 32610632
Test: manual inspection of /proc files to verify group.
Test: manual inspection of zygote's children to make sure they do not
      inherit AID_READPROC

Change-Id: I3619a9ae33c8077e068e8024f7c7d44cfca6fb76