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: I74b4a16266bda7b8ac740b3a0193268da260fc2f
Change-Id: I1c2fe707d865e6a1953b4ab82194d4b9132258f9