Refactoring to ensure item deletion always removes from the DB.

- Routing the various places where we call through to delete from
  LauncherModel through Launcher, which will delegate the removal
  of the icon from the workspace, and properly handle the removal
  of all items and their contents from the db.

Bug: 23944119

Change-Id: I022fe2b3e79da16b5af87505c4362490b8422686