[PT16] Simplification of sendProxyBroadcast.

sendProxyBroadcast is always called with the same argument, and
it would make no sense with another argument anyway. Remove it.

This concludes the ProxyTracker refactoring with 227 lines removed
from ConnectivityService, a lot clarified, and some bugs removed.
Things can still be improved, but presumably at a much higher cost.

Next steps are : write tests, now that ProxyTracker is both testable
and mockable. And try to pour some gasoline on the PROXY_CHANGE_ACTION
broadcast, see if it burns well.

Test: runtest
Change-Id: I66e40b4bf5cfd0b2dc4fa37ea97b3429fe1b7e6c