Bug 910189: Part 1 - Add "restore defaults" hooks into settings. r=margaret

--HG--
rename : mobile/android/base/resources/xml/preferences_search.xml => mobile/android/base/resources/xml-v11/preferences_search.xml
Bug 910189: Part 2 - Enable removing of default engines. r=margaret
Bug 910189: Part 3 - Restore default search engines (no dialog). r=margaret