Add OWNERS for VTS module VtsHalGraphicsMapperV2_1Target

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: I871c2ca756593107e75a1265e351644062a3ac5d