Background data notification, API clean up.

When restricting background data, show ongoing notification to give
easy access to re-enable. Deprecate getBackgroundDataSetting() API
to always return true, since NetworkInfo.isConnected() is new source
of truth. Handle upgrade path by reading from existing secure value,
and kick one last broadcast when changing value. Remove background
data code from ConnectivityService.

Remove warning alerts, since they push ifaces into restricted list;
should only happen when iface has limit.

Bug: 5163559, 5129421
Change-Id: I0064d9d643656a4d32aaae51d4a58bce49fe295f