sed "-i" flag on Mac has different syntax than on Linux. Replace use of
sed with grep.

A simple fix like this should suffice for this case, but ideally, we
should maintain our own utils instead of using tools on the host
machine.

Fixes: 121235932
Test: m selinux_policy
Change-Id: I46c3bdb90bf7de48d2c942b15a65ce82ae3041c5