Always trigger prop change event for WAIT_FOR_VHAL.

CarService always expect VHAL to generate a property change event
when sending WAIT_FOR_VHAL even though there is not actual power
state change in VHAL.

VHAL power state might already be ON when receive WAIT_FOR_VHAL if
CarService client crashes and restarts. In this case, CarService
still expects an event to be generated.

VehiclePropStore would only generate event when there is an actual
property value change, so we must erase the value before setting
the value to ensure a change event.

Test: atest FakeVehicleHardwareTest
Bug: 219861737
Change-Id: I185b07e188f0b581253b5ab29faf859247ed9d38