Add default code reviewers into OWNERS

Bug: 33166666
Test: gerrit uploader
Change-Id: Ifb9629a958e1501f98686d6ea1236afa49101d3e

OWNERS

+# Default code reviewers picked from top 3 or more developers.
+# Please update this list if you find better candidates.
+riggle@google.com
+olorin@google.com
+danielaustin@google.com