Add X-CS response for all requests that came from Zero network

All mobile responses need to have the X-CS set if it came from a zero network.
This allows apps to detect when we switch from zero to non-zero and back,
or when the user switches from one network to another and both are zero (multi-sim phones).
The client should constantly check each response for this header, and if it changes,
re-request the partner's message info.

Change-Id: If3dd1115148c3ea0a0e4bdf79c11343d0d21ac44