Fixing missing updates in package install sessions

> Ensure icon cache never returns null icon
> Enabling install shortuct receiver only after workspace has finished binding
> Making all the model changes for package installs on worker thread and only
posting the updaes on the UI
> Making shortcut exists check on the loaded items and not on the DB
> Explicitely using worker thread for PckageInstallListener
> Removing backward compatibility support from PackageInstallerCompat

Change-Id: I9592771b9670c1c1c84c8208cae8dafa7b393e65