Fix several HTTP proxy issues with multinetworking.

1. Send PROXY_CHANGE_ACTION broadcast when any network's proxy changes,
   not just the default network.
2. When a process is bound to a particular Network, update the proxy
   system properties to those for the bound Network, and keep them
   updated when PROXY_CHANGE_ACTION broadcasts are received.
3. Make Network.openConnection() use the proxy for the Network.

bug:17905627
bug:17420465
bug:18144582

(cherry-pick of https://android-review.googlesource.com/#/c/115170)

Change-Id: Ia2819985e6108a8c121e74c683a5646becfd0a97