Add OWNERS file for libhostgraphics code

This library is used to provide customization of Android graphics code
for building on host. This adds as owners the owners of HWUI as it is
the most closely related library, plus the maintainers of layoutlib
(which is the reason for libhostgraphics existence).

Bug: 322360037
Test: N/A
Change-Id: Ic163a0164e4d1da3d7cb1853fb69821b35d3d08e