Make FakeVehicleHardware async.

Use a handler thread to handle all the get/set value requests in
FakeVehicleHardware to make it async. Vendor implementation should
use similar logic to make their VHAL async.

Test: atest FakeVehicleHardwareTest
Bug: 226001897
Change-Id: Iba95125e63566c27597b9d11bff131c229846d49