Back button in search always goes back or closes search

Before, the back button had different behavior depending on
which part of the search dialog had focus, and depending on
whether there were more suggestions than could fit in the
visible space in the drop-down.

This change makes the behavior of the BACK button more
predictable, by always going back to QSB when pivoted
into a source, and closing the search dialog when in
QSB.

Fixes http://b/issue?id=2126526
"One tap of back button when entering text to search
should always take you back to previous page"

Change-Id: I7b3553122eaa1a5d87ac966d5ab2a9f432063cc9