Use formal API for ActivityThread to set proxy

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

getProxyForNetwork allows clients to find which proxy applies, which can
be a global proxy setting or a per-network proxy.

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
Change-Id: Ifb516194ecde1567cea4b6806946091cdcf2f015