ShortcutManager: Make sure persisted default launcher still exists.

- Also make sure to ignore unexported activities.
(e.g. unexported activities shouldn't have shortcuts.)

- Also add unit tests for package manager related operations.

- Also remove stale TODOs (per-activity shortcut count check is
implemented already.)

Bug 29699769
Bug 29516954

Change-Id: Ia18301baf6bec1ad71ae195d9ae3d10bd8386fc4