Document limitations of the VpnService setHttpProxy API

There is no way to make proxies work on split tunnels in the
current architecture, because only the system knows about
the split tunnels but it's the app that sends requests to
the proxy.

Test: comment-only change
Bug: 185574854
Change-Id: Ica7096c31e5e81373b80ac5eef92d82783a38ed5
1 file changed