Freeze Networkstack AIDL API as V6

This APIs needs to be frozen before release,
because unstable version in REL will be disallowed.

Bug: 133526962
Test: m
Original-Change: https://android-review.googlesource.com/1277432
Merged-In: I01096d4fb9f60b675aae5c3fe5d5a49d9ae0775c
Change-Id: I01096d4fb9f60b675aae5c3fe5d5a49d9ae0775c