Copy SearchSourceSelector from QuickSearchBox

The widget shows the icon for a search source, and when clicked
fires an intent that shows a search source selection activity.
That intent is handled by the QuickSearchBox app.

This change also adds the source selector to the in-app
search dialog.

An upcoming change to QuickSearchBox will add the search source
selector to the home screen search widget and to the Quick Search Box
activity.

TODO: Add assets for selected and pressed states to the search selector.
TODO: The SearchDialog hides when the soruce selection activity
appears. This will be fixed when SearchDialog is changed from a
system window to a normal app window.

Change-Id: I91eadd60682577614e274ecf5b995b927c70a48a