Update AUTOMATIC_EMERGENCY_BRAKING_ENABLED documentation in HAL

An implementation of AUTOMATIC_EMERGENCY_BRAKING_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: 264336118
Test: atest VtsHalAutomotiveVehicle_TargetTest
Change-Id: I2dab0c48865ed1ccc40433b9fbe5ea341a2488f3