ash: Rename LauncherModel to ShelfModel.

* Move launcher_model.* from launcher/ to shelf/
* Rename to shelf_model.*

BUG=248353
TEST=None, no functional changes
R=jamescook@chromium.org,harrym@chromium.org

Review URL: https://codereview.chromium.org/71653003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235217 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235217