Fix HttpUrlConnection#isUsingProxy.

The earlier implementation disregarded what the
ProxySelector had to say. We now query the selected
route (if one has been established).

Fixes compatibility test:
HttpURLConnectionTest#testUsingProxySelector

(cherry picked from be9eef71)

bug: 10209026

Change-Id: I3183c50ce21c5a19ac1f34f2ff5648683b526dd7