Better OWNERS for system_messages.proto

This file notification ids to be used anywhere in the android package (e.g. system_server). Match platform resource OWNERS that has similar scope.

Change-Id: I2d525d59c24df63429ef090e41e45952506c1c59

proto/src/OWNERS

 per-file gnss.proto = file:/services/core/java/com/android/server/location/OWNERS
 per-file wifi.proto = file:/wifi/OWNERS
 per-file camera.proto = file:/services/core/java/com/android/server/camera/OWNERS
+per-file system_messages.proto = file:/core/res/OWNERS