Create ConnectedClient to manage VHAL clients.

Create a class to represent each VHAL client for each operations.
This class would manage the pending requests and maintain callbacks
that should be called when we get the result for async requests or
when the request time-out.

Test: atest DefaultVehicleHalTest
Bug: 200737967
Change-Id: I0e0be46f4a77b5dd5569fa949f67618a9aa9ac73