Implement Unsubscribe in vhal proxy.

Implement the unsubscribe function in IVehicleHardware. This will
stop the server from generating property update events for the
specified [propId, areaId].

Test: atest GRPCVehicleHardwareUnitTest GRPCVehicleProxyServerUnitTest
Flag: EXEMPT hal change
Bug: 328316981
Merged-In: I35f4860eead0c8ec9b192657fe51cc0ff4319383
(cherry-picked from commit: cb00b1f816670c291044087db9f88cb76a2a9b3e)

Change-Id: I35f4860eead0c8ec9b192657fe51cc0ff4319383
diff --git a/automotive/vehicle/aidl/impl/grpc/GRPCVehicleHardware.h b/automotive/vehicle/aidl/impl/grpc/GRPCVehicleHardware.h
index bec957c..9750f62 100644
--- a/automotive/vehicle/aidl/impl/grpc/GRPCVehicleHardware.h
+++ b/automotive/vehicle/aidl/impl/grpc/GRPCVehicleHardware.h
@@ -85,6 +85,8 @@
 
     aidlvhal::StatusCode subscribe(aidlvhal::SubscribeOptions options) override;
 
+    aidlvhal::StatusCode unsubscribe(int32_t propId, int32_t areaId) override;
+
     bool waitForConnected(std::chrono::milliseconds waitTime);
 
   protected: