Return config using a copy not a pointer.

Introduce VehiclePropertyStore.getPropConfig that returns a copy of
the config instead of a constant pointer. If the internal map is updated,
the pointer might become invalid, so it is safer to just return a copy.

We do not modify the existing API for backward compatibility.

Test: atest VehiclePropertyStoreTest
Bug: 308202443
Change-Id: I769866a09577cc69d25276349b7688cabcbc0c20