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/update_engine.xml b/update_engine.xml
index f5faaeb..9cd9d42 100644
--- a/update_engine.xml
+++ b/update_engine.xml
@@ -16,6 +16,10 @@
     </method>
     <method name="CheckForUpdate">
     </method>
+    <method name="AttemptUpdate">
+      <arg type="s" name="app_version" />
+      <arg type="s" name="omaha_url" />
+    </method>
     <signal name="StatusUpdate">
       <arg type="x" name="last_checked_time" />
       <arg type="d" name="progress" />