Add support to update_engine_client for -app_version and -omaha_url.

These options prevent auto-detection of these parameters.
Note that this CL makes the check_for_update DBus method obsolete from
the client's point of view.

BUG=4593
TEST=unit tests, gmerged on device and tried the client with different options.

Review URL: http://codereview.chromium.org/3048008
diff --git a/UpdateEngine.conf b/UpdateEngine.conf
index fe332c0..05f0056 100644
--- a/UpdateEngine.conf
+++ b/UpdateEngine.conf
@@ -15,10 +15,13 @@
     <!-- allow explicit methods -->
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"
-           send_member="GetStatus"/>
+           send_member="AttemptUpdate"/>
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"
            send_member="CheckForUpdate"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
+           send_member="GetStatus"/>
   </policy>
   <policy context="default">
     <deny send_destination="org.chromium.UpdateEngine" />