Add subscribe/unsubscribe to IVehicleHardware.

These two functions replaces updateSampleRate. Previously
updateSampleRate will be called when a continuous property is
subscribed/unsubscribed. However, IVehicleHardware layer does not
know when an on-change property is subscribed/unsubscribed. This CL
introduces two new functions to notify IVehicleHardware when any
properties are subscribed/unsubscribed.

Test: atest DefaultVehicleHalTest
Bug: 306262618
Change-Id: I8d32d1eb919036015b5082f74e259bcd4b1bd29e