Check for Ignore-AOSP-First commit tag

If the Ignore-AOSP-First tag is in a commit message, it means the
committer is intentionally bypassing the AOSP source of truth. Check
for this tag in the AOSP-first presubmit hook so that devs don't have
to turn off hooks to get past the AOSP-first policy in exceptional
cases.

Bug: 189139990
Change-Id: I18231b93d7f213c13ace0a0af24bb20a394cc50f
Test: Manual testing with various commit messages