Added focusable true on search_button for D-pad.

To give focus to search_button on ActionBar by using D-pad,
the search_button should be set focusable true.

How to reproduce:
(1) Open "API Demos" application -> App -> Action Bar -> Action Bar Usage
(2) Try to focus the Search View, using the D-pad.

Change-Id: Idc0b8e8d88391c152e0fc20467c23d03ab558f62