Use unstable networkstack AIDLs in development branches

Development branches should use the -unstable version of the
NetworkStack AIDLs so that refreezing is not necessary for each
modification.
The versions will be re-frozen before each release instead.

Bug: 157534516
Test: m
Merged-In: Iae5eb6d6a7ea38cd8e2537f1eb83190ecbae1f87

Change-Id: Iae5eb6d6a7ea38cd8e2537f1eb83190ecbae1f87