Use references in GATT client/server API (2/3)

This allows to get rid of some unnecessary null checks, and guarantee
proper values are passed.

Test: compilation test
Change-Id: I1b790dba0cfc3ab02984c3911d83a6f413a1e7e6
diff --git a/service/gatt_server.cc b/service/gatt_server.cc
index 79d9c35..d2550cd 100644
--- a/service/gatt_server.cc
+++ b/service/gatt_server.cc
@@ -159,7 +159,7 @@
   bt_status_t result =
       hal::BluetoothGattInterface::Get()
           ->GetServerHALInterface()
-          ->send_response(connection->conn_id, request_id, error, &response);
+          ->send_response(connection->conn_id, request_id, error, response);
   if (result != BT_STATUS_SUCCESS) {
     LOG(ERROR) << "Failed to initiate call to send GATT response";
     return false;
@@ -583,7 +583,7 @@
       hal::BluetoothGattInterface::Get()->GetServerHALInterface();
   bt_uuid_t app_uuid = uuid.GetBlueDroid();
 
-  if (hal_iface->register_server(&app_uuid) != BT_STATUS_SUCCESS) return false;
+  if (hal_iface->register_server(app_uuid) != BT_STATUS_SUCCESS) return false;
 
   pending_calls_[uuid] = callback;