Add subscription manager.

Add a class to manage VHAL subscription. It supports subscribing
and unsubscribing to properties. For continuous property, it uses
recurrent timer to recurrently calls the registered function. For
on-change property, it would return all subscribed clients for a
given property.

Test: atest DefaultVehicleHalTest
Bug: 200737967

Change-Id: I3e1a0401fd465dc31fe08ea77d5a6651fa7bbfaf