Don't use dim behind for the search box.

This causes ugly artifacts, such as when dismissing, the full white
completions window disappears into the dimmed background and then
the dimmed background immediately starts undimming.  Likewise when
being displayed, the content behind dims down and then the full
white completions window appears on top.

Change-Id: Id33fb7a172796412143d84e4b3d8915624565f7d