Fix interfaceVersion in DhcpServerCallbacks

The getInterfaceVersion method was missed when freezing the AIDL
interface version.
This causes Q devices to return 0 instead of 3 on this interface.

Bug: 139110717
Test: toggle tethering, dumpsys network_stack version
Change-Id: If587cc9ead4191b9c2ad2ae65473bd89e7c37b5c