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
077357ccf0