[CrOS Tether] Add KeepAliveTickleResponse messages to both the proto file and MessageWrapper.

chromeos/components/tether/proto/tether.proto

Index: chromeos/components/tether/proto/tether.proto
diff --git a/chromeos/components/tether/proto/tether.proto b/chromeos/components/tether/proto/tether.proto
index a0b7ddf1f3d593770519aab6368749ff789249a8..634429c86add89b7b496a708f648bc2ab334a136 100644
--- a/chromeos/components/tether/proto/tether.proto
+++ b/chromeos/components/tether/proto/tether.proto
@@ -19,6 +19,7 @@ enum MessageType {
   CONNECT_TETHERING_RESPONSE = 4;
   DISCONNECT_TETHERING_REQUEST = 5;
   KEEP_ALIVE_TICKLE = 6;
+  KEEP_ALIVE_TICKLE_RESPONSE = 7;
 }
 
 // Meant to communicate the host's current Wifi status. This is intended to
@@ -103,5 +104,10 @@ message ConnectTetheringResponse {
 // Next id: 1
 message KeepAliveTickle {}
 
+// Next id: 2
+message KeepAliveTickleResponse {
+  optional DeviceStatus device_status = 1;
+}
+
 // Next id: 1
 message DisconnectTetheringRequest {}