Don't consume all BACK keys in SearchView

If SearchView is the first focusable, it will always get focus
(in non-touch-mode) when it tries to clearFocus on BACK pressed.
This lead to a situation where SearchView always consumed the
BACK key leaving users unable to leave some activities.

It looks like this was done so that pressing back both closed the
auto-correct popup AND the ime (whereas without reimplementing
onPreIme, it would require 1 back-press to close the auto-correct
popup and then a subsequenty press to close the IME). It also,
however, tried to clearFocus as well.

This change only consumes the Back press if the auto-correct popup
is open (to have the same effect of BACK closing both the popup
and the IME at the same time). It ignores the back press otherwise.
If there is no pop-up, this results in the BACK being handled by
the ime and thus hiding it. Otherwise, back is not consumed.

The only effective difference is that the SearchView remains
focused now.

Bug: 73181998
Test: SearchView CTS tests still pass. Back can now exit test-app
      once IME is hidden.

Change-Id: I3fe687b5344300b86131f44a1c9798cd736955bd