system-proxy: Notify proxy worker active

This CL implements the |WorkerActive| dbus signal. Clients subscribed
to this event can get the address of the local proxy from the signal
parameters.

It also fixes a couple of other issues:
- naming the ShutDown method in the dbus conf file
- formatting the target URL sent to Chrome for proxy resolution

BUG=chromium:1042642
TEST=unittests, manual test on DUT

Change-Id: I8dd33e9c3be7ddb2669c155850dcb02ba37aae4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2190633
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Tested-by: Andreea-Elena Costinas <acostinas@google.com>
Commit-Queue: Andreea-Elena Costinas <acostinas@google.com>
diff --git a/system-proxy/server_proxy_test.cc b/system-proxy/server_proxy_test.cc
index c69afcb..34824e7 100644
--- a/system-proxy/server_proxy_test.cc
+++ b/system-proxy/server_proxy_test.cc
@@ -184,7 +184,7 @@
   ASSERT_TRUE(ReadProtobuf(stdout_read_fd_.get(), &request));
   ASSERT_TRUE(request.has_proxy_resolution_request());
 
-  EXPECT_EQ("www.example.server.com:443",
+  EXPECT_EQ("https://www.example.server.com:443",
             request.proxy_resolution_request().target_url());
 
   EXPECT_EQ(1, server_proxy_->pending_proxy_resolution_requests_.size());