Add getInterfaceVersion to NetdEventListener

This is necessary to freeze netd_event_listener_interface.

Bug: 133124190
Test: Built with change, new GtsNetworkStackHostTestCases passes
(cherry-pick from aosp/968213)
Merged-In: I12c7bd86b796964fff658a326e99e55149e47809
(cherry picked from commit 9ef856dc)

Change-Id: I9928b10718f5c9ebcfd98d0fdee184b9cb691f6a