apps/bttester: refactor GATT service

Accomodated to new API.

apps/bttester/src/btp/btp_gatt.h

@@ -128,6 +128,9 @@ struct btp_gatt_descriptor {
 } __packed;
 
 #define BTP_GATT_EXCHANGE_MTU        0x0a
+struct btp_gatt_exchange_mtu_cmd {
+    ble_addr_t address;
+} __packed;
 
 #define BTP_GATT_DISC_ALL_PRIM_SVCS        0x0b
 struct btp_gatt_disc_all_prim_svcs_cmd {
@@ -305,7 +308,7 @@ struct btp_gatt_get_attribute_value_rp {
 } __packed;
 
 #define BTP_GATT_CHANGE_DATABASE        0x1e
-struct btp_gatt_change_database {
+struct btp_gatt_change_database_cmd {
     uint16_t start_handle;
     uint16_t end_handle;
     uint8_t visibility;

apps/bttester/src/btp/bttester.h

@@ -101,9 +101,6 @@ tester_init_gatt(void);
 uint8_t
 tester_unregister_gatt(void);
 void
-tester_handle_gatt(uint8_t opcode, uint8_t *data,
-                   uint16_t len);
-void
 tester_handle_gattc(uint8_t opcode, uint8_t *data,
                     uint16_t len);
 int