Make search UI open up quicker to the user.

On invoking the search UI, the drop down list box with past queries and shortcut items
appears after a few hundreds of milliseconds on screen. This was because we were
displaying the drop down within the onFilterCompleted callback after the list box filtered
the items based on the given query text. While that code path is necessary, for the first
invocation of the search dialog with an empty query we can show the list box before the
text stuff happens. This change does that, issuing an async request to show the drop
down after pending events have been processed.

Without this change, on an average it took 350ms for the code to show the drop down.
With this change, on an average it takes 150ms.