Use IBinder to identify callbacks in VHAL

Right now there's no way to distinguish multiple clients, thus we cannot
associate IVehicleCallback from subscribe and unsubscribe method. As a
temporary solution we will use pointer to IBinder from underlying proxy
object until HILD will have some API to do that.

Test: verified unit tests run, checked that subsequent 'subscribe' calls
do not create new client objects in VHAL

Bug: b/37254245
Change-Id: I029564b7b03e4821287c2110d0929a5c4cbce0ce