Add IconCache support for deep shortcuts, loads deepshortcut on

background thread.  Added a feature flag to toggle on/off this
feature.

Bug: 140242324
Test:
  1. (Custom Shortcut) Long click on google maps -> widgets -> drag
     driving mode to workspace.

  2. Open chrome -> add to home screen -> add -> add automatically.

  3. InstallShortcutReceiver
     In Launcher.completeAddShortcut, commend out the code that
     calls PinRequestHelper.createWorkspaceItemFromPinItemRequest,
     then open chrome -> add to home screen -> add -> add
     automatically.

  4. ShortcutDragPreviewProvider
     qdb -> long press on suggested app that has deep shortcut
     -> drag to workspace.

Change-Id: If7babe4eddf5434909bf686b4e9bde15e444d9fd