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/UpdateEngine.conf b/UpdateEngine.conf
index 2149798..534fade 100644
--- a/UpdateEngine.conf
+++ b/UpdateEngine.conf
@@ -21,7 +21,13 @@
            send_member="GetStatus"/>
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"
+           send_member="GetTrack"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
            send_member="RebootIfNeeded"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
+           send_member="SetTrack"/>
   </policy>
   <policy context="default">
     <deny send_destination="org.chromium.UpdateEngine" />