Refactoring ShortcutManager + bug fixes.

- Don't pass the ShortcutService instance as an argument.  This tiny
optimization is no longer meaningful now that PackageShortcut and
PackageLauncher have reference to ShortcutUser.

- Rename mLauncherComponent to mDefaultLauncherComponent for clarity.

- Don't instantiate ShortcutPackage instances when not needed.

- Don't allow intents with a null action.

- Also improve javadoc.

Bug 28592642
Bug 28474517
Bug 28557169

Change-Id: I8790d3494bf3b92c143c02824b0ed0e514504baa