Move to a single dropdown background for the search dialog. This one

is full-screen width but has rounded corners on the bottom so it doesn't
look crappy if the list is not full-screen height. This was already
being used for in-app search, so I just got rid of the old global search
background and use the in-app one for everything.

By removing the lines in SearchDialog, I'm causing it to rely on the
value specified for the dropdown background in
frameworks/base/core/res/res/layout/search_bar.xml.