Conditionally disable NetworkStackApi33Shims

The "current" shim targets are only necessary in branches that can use
non-stable APIs, and can fail to build otherwise.

Apply ConnectivityNextEnableDefaults so they are disabled on such
branches. Also deprecate NetworkStackNextEnableDefaults, so
ConnectivityNextEnableDefaults is used everywhere.

Bug: 190249673
Test: m
Change-Id: Iffa76a05a803e0c91fa81170f4bc5f2022d1514f