Undeprecate android.net.ProxyInfo as it's still used.

It was deprecated as part of the Apache deprecation, however it is used
by ConnectivityManager.getLinkProperties() and .getDefaultProxy().
The alternatives (e.g. java.net.Proxy) do not offer a complete
replacement as they lack certain fields (e.g. PAC URL).

Bug:21274137
Change-Id: I99163a2611a738f337fb9907349ef6255dbed6d6