use .prop() to manipulate checked state in search box

To manipulate the checked state of a checkbox we need to modify the
"checked" DOM property, not attribute, as explained here:

http://api.jquery.com/prop/

The .attr() vs. .prop() distinction was added in jQuery 1.6 so our
previous version of jQuery 1.5 was presumably doing the right thing
behind the scenes with attributes.

Bug: 1010624
Change-Id: I07b29c55dbaa491e09bfbd6f1b9d0630c6b58015