theme: adds padding to search-provider-icon

Adds 15px padding to all sides of provider icon to have padding
which seems equal to that of list-search-result-content. This aligns the
provider icon vertically with the search result content.

Padding is set to 15px as list-search-result-content has 12px padding
and the outer box (list-search-result) has 3px.

http://bugzilla.gnome.org/show_bug.cgi?id=695760