Checking for unlauncheable apps.

When loading apps from prefs, detecting apps that cannot
be launched (for example, were uninstalled) and not adding them
to the shelf.

Also watching for apps that become unlauncheable while the shelf
is shown, and removing them as it happens.

Bug: 22694757
Change-Id: Iab9494f0480e9bf4438b72a94041002be010334f