Add default code reviewers into OWNERS

Bug: 33166666
Test: gerrit uploader
Change-Id: I7c3d47abe31017eafafa7393f96bfb268b76f81a

OWNERS

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