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.

Fixes: 139110717
Test: toggle tethering, dumpsys network_stack version
Merged-In: I782f479de2d7d4c6579f206645a580c905009d85
(cherry picked from commit c6c1c693)

Change-Id: I16b08e77351399d937b037c700b6e454b09f6d16