Add default and nullable annotations to NetworkStack AIDLs.

All new fields should have defaults. Do that for fields added in
v6 before we freeze v6. Fields added in v5 can no longer be
changed because v5 is frozen.

Test: m
Bug: 133526962
Change-Id: Ie2ecd84919a59adf89d0c56e4fdf2b2258fa1448