Add an OWNERS_core_networking_xts for CTS/GTS/... and use it.

This uses a similar scheme to our existing OWNERS files, which
all come from a common location in the master branch.

Test: repo upload
Change-Id: I672070302789ac38a738ab6626f47ec9cbc75dd9

OWNERS_core_networking_xts

+lorenzo@google.com
+satk@google.com

tests/cts/OWNERS

 # Bug component: 31808
 set noparent
-lorenzo@google.com
-satk@google.com
\ No newline at end of file
+file:platform/packages/modules/Connectivity:master:/OWNERS_core_networking_xts