Use formal API for ActivityThread to set proxy

Add setHttpProxyConfiguration to the public API, and use
ConnectivityManager APIs from ActivityThread (instead of hidden APIs) to
get/set the proxy for an app process.

The default proxy is now initialized with getDefaultProxy instead of
getProxyForNetwork(null); this should not make a difference, as nothing
should have called bindProcessToNetwork at that point yet.

Bug: 174436414
Test: m; device boots
Merged-In: Ifb516194ecde1567cea4b6806946091cdcf2f015

Change-Id: I06b797eeae54609aecdc0afe1df4e6c602a17a69