Fix malformed OWNERS.

Per go/android-owners, include directions should be of the form
[gerrit_project_name]:[absolute_path_to_included_file].

Exempt-From-Owner-Approval: Only editing OWNERS file that ought
to be owned by ourselves, anyway.

Bug: 159055442
Test: Treehugger
Change-Id: I9130985b484349264f1fc42c320e93b2bc672a59