Add PendingRequestPool to handle pending requests.

PendingRequestPool would store all pending requests that we have
not yet got responses from hardware. If a request has been pending
for too long, the timout callback would be called and the request
would be removed.

Test: atest DefaultVehicleHalTest
Bug: 203713317
Change-Id: I4d7ae2c72b960347be70ac4cc8ce3d66eb8128f9