Fix bug #15460001 "Cell broadcasts" setting search suggested in wifi only...

Fix bug #15460001 "Cell broadcasts" setting search suggested in wifi only device though setting is not present

- implement a BaseSearchIndexProvider for WirelessSettings
- fix Context passed to the Index: now use an ApplicationContext
- also use the correct Context when dealing with Settings SearchIndexProvider
- also makes impossible a rogue BaseSearchIndexProvider to crash Settings
if not well behaving
- last, fix icon used for "NFC & more" search results

Change-Id: I8b5cc999fa57a9e1977ee57394801d54cf3dbabb