Fix presubmit for hidden api.

grep returns 1 for if nothing was found. This fails presubmit hook due
to bash handling "set -e" Looks like grep not found is valid case.
This fixes this issue by bypassing grep exit code.

Test: Locally
Bug: 161351699
Change-Id: I39e926a348a0fb8f4040f86034affdb4204faac5