Add BUILD_BROKEN_USES_NETWORK

Some people apparently still talk to the network during their build.
Allow this temporarily with a BUILD_BROKEN_USES_NETWORK check.

Bug: 129992021
Test: attempt to talk to the network during the build with and without
      this flag.
Change-Id: Ifb967c656aa24c4599e7232d0f1b5a303b5bac52