AU: Implement getting of tracks through GetTrack.

Add SetTrack and GetTrack to UpdateEngine.xml as well.

BUG=chromium-os:8104
TEST=tested on device with update_engine_client

Change-Id: I10ef2552e9280524b3a8d5f232d104a81a114e06

Review URL: http://codereview.chromium.org/4181001
diff --git a/update_engine.xml b/update_engine.xml
index 73bc92e..2008d2d 100644
--- a/update_engine.xml
+++ b/update_engine.xml
@@ -18,6 +18,9 @@
       <arg type="s" name="new_version" direction="out" />
       <arg type="x" name="new_size" direction="out" />
     </method>
+    <method name="GetTrack">
+      <arg type="s" name="track" direction="out" />
+    </method>
     <method name="RebootIfNeeded">
     </method>
     <method name="SetTrack">