Update ADAS ENABLED properties documentation if property unavailable

An implementation of AUTOMATIC_EMERGENCY_BRAKING_ENABLED or FORWARD_COLLISION_WARNING_ENABLED should return false when there's an issue with the ADAS system and it cannot retrieve
the property. It should not return NOT_AVAILABLE.

Bug: 264946034
Test: atest VtsHalAutomotiveVehicle_TargetTest
Change-Id: Ieddbcdf766543cfb94cb5b295b22ac4369641c81