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/GRPCVehicleProxyServer.h b/automotive/vehicle/aidl/impl/grpc/GRPCVehicleProxyServer.h
index 4f0c1e4..dd9e2aa 100644
--- a/automotive/vehicle/aidl/impl/grpc/GRPCVehicleProxyServer.h
+++ b/automotive/vehicle/aidl/impl/grpc/GRPCVehicleProxyServer.h
@@ -60,6 +60,10 @@
     ::grpc::Status Subscribe(::grpc::ServerContext* context, const proto::SubscribeRequest* request,
                              proto::VehicleHalCallStatus* status) override;
 
+    ::grpc::Status Unsubscribe(::grpc::ServerContext* context,
+                               const proto::UnsubscribeRequest* request,
+                               proto::VehicleHalCallStatus* status) override;
+
     ::grpc::Status CheckHealth(::grpc::ServerContext* context, const ::google::protobuf::Empty*,
                                proto::VehicleHalCallStatus* status) override;