Clear the text in the box every time a search is done.

Vibha did some user testing and found that some people have trouble figuring
out how to search again once they've done an initial search. This could be
because the text you search for remains in the text box after you've searched.
Since there's not a particularly good reason to leave that text in there, this
patch clears the search box every time someone navigates to a new page.

Change-Id: I33ad2a6cbe2992f0306b13588936ca4e28c4102a
Merge "Clear the text in the box every time a search is done."