[Android] Fix back arrow in SiteSettings

Clearing the query means assigning an empty string.
In SiteSettings, the empty string means that a search for the empty
string is active. To really close a search, |null| must be assigned.

This means before this CL, the resumed activity would clear the search
by assigning an empty string and with that, leave a (hidden) search
open. This caused the back arrow to seemingly have no effect as the
first tap on the icon only closed the hidden search.

Now, SiteSettings set the query to null after clearing it and the
back button has immediate effect.

Bug: 879532
Change-Id: If45855db7309859fd0c63ec286048cc047281b18
Reviewed-on: https://chromium-review.googlesource.com/1202302
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Friedrich Horschig [CEST] <fhorschig@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#588529}(cherry picked from commit 07aecde)
Reviewed-on: https://chromium-review.googlesource.com/1207570
Reviewed-by: Friedrich Horschig [CEST] <fhorschig@chromium.org>
Cr-Commit-Position: refs/branch-heads/3538@{#49}
Cr-Branched-From: 79f7c91-refs/heads/master@{#587811}

Author: FHorschig

chrome/android/java/src/org/chromium/chrome/browser/preferences/website/SingleCategoryPreferences.java

@@ -540,6 +540,7 @@ public void onResume() {
 
         if (mSearch == null && mSearchItem != null) {
             SearchUtils.clearSearch(mSearchItem, getActivity());
+            mSearch = null;
         }
 
         getInfoForOrigins();