Use resource ID for searchable description

Before, SearchableInfo read and cached the localized
searchable description, which meant that it was not updated
on locale changes. Now SearchableInfo only holds the resource
ID.

SearchableInfo is a new API in Froyo, so it's ok to change.

Part of http://b/issue?id=2175247

Change-Id: I1898f7895b9172f58419d906ad741cb7dd1e7252