fw/b data restrictions: Don't call getNetworkCapabilities() in the callback

* Docs say that calling getNetworkCapabilities() from within a network
  callback is racy and not to do it.

* Refactor to make use of onCapabilitiesChanged() to glean capabilities
  instead.

Change-Id: If9c4cd7c1bd0594697b0ac98903600ecd583e55b