Add OWNERS for VTS module vts_libdm_test

Each VTS module is required to have OWNERS file. The ownership is based on
go/vts-owners. For more information about ownership policy, please visit
go/xts-owners-policy.

Test: Tree Hugger
Bug: 143903671
Change-Id: Id2911fd62841bf5622607f9deeb50c7cf1413b1b