Use AIDL for test vendor properties.

Use AIDL enum instead of hard coding the test vendor property
definitions so that we don't have to duplicate the value among VHAL
and the clients. This also shows how vendor can uses a custom AIDL
enum to define vendor properties.

Test: atest FakeVehicleHardwareTest
Bug: 296913406
Change-Id: Ief889b86bfbef136f8afd4a4f49405f2213b441c