Move android.server.search.SearchableInfo to android.app

We need to expose SearchableInfo in the SDK in order to unbundle
Quick Search Box. Since the android.server.search package is
hidden, I'm moving SearchableInfo to android.app, where
SearchManager lives.

This change doesn't actually expose SearchableInfo. I'll do
that in a separate change to keep the change that
api-council needs to review small.

This is part of the fix for: http://b/issue?id=2270838

Change-Id: I9589f9c2c11d36c958beedff8245fe0c3319c6ba