Support Bluetooth keyboard navigation in Search Engine settings

Problem: Currently unable to navigate the Search Engine settings
menu via (Bluetooth) keyboard navigation (up/down/tab/enter keys).
This is because the ListItems in the ListView (contains search
engine choices) are not focusable.

Solution: Make Search Engine settings options focusable, so that
they can navigated and receive click events with a keyboard
(up/down/tab/enter keys).

Bug: 936992
Change-Id: Iee972f689bc497f4393743c0132489715946ea83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1495804Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Simon La Macchia <smacchia@amazon.com>
Cr-Commit-Position: refs/heads/master@{#637961}

chrome/android/java/res/layout/search_engine.xml

@@ -9,6 +9,7 @@
     android:background="?android:attr/selectableItemBackground"
     android:gravity="center_vertical"
     android:orientation="horizontal"
+    android:focusable="true"
     android:padding="6dp">
 
     <android.widget.RadioButton

chrome/android/java/src/org/chromium/chrome/browser/preferences/SearchEnginePreference.java

@@ -47,6 +47,7 @@ public class SearchEnginePreference extends PreferenceFragment {
         mListView = (ListView) getView().findViewById(android.R.id.list);
         mListView.setAdapter(mSearchEngineAdapter);
         mListView.setDivider(null);
+        mListView.setItemsCanFocus(true);
     }
 
     @Override