AU: Implement switching of tracks through SetTrack.

On official images, update engine allows updating the track
only to dev-channel or beta-channel. The track is verified
both at setting and at getting time.

BUG=8104
TEST=unit test; tested on device

Change-Id: Ic81d4e3a9e09554d2239ee5c7a6c78e4dfe30d19

Review URL: http://codereview.chromium.org/4103002
diff --git a/update_engine_client.cc b/update_engine_client.cc
index a662242..20c86a5 100644
--- a/update_engine_client.cc
+++ b/update_engine_client.cc
@@ -189,7 +189,7 @@
                                                    &error);
   CHECK_EQ(rc, true) << "Error setting the track: "
                      << GetGErrorMessage(error);
-  LOG(INFO) << "TODO: Track permanently set to: " << track;
+  LOG(INFO) << "Track permanently set to: " << track;
 }
 
 static gboolean CompleteUpdateSource(gpointer data) {