Shortcut: Only "main" activities can have shortcuts.

- Don't publish shortcuts when their target activities are not main.
- Only scan manifest shortcuts for main activities.
- When an app is updated, remove shortcuts that no longer belong to
valid main activities.

- Also re-publish manifest shortcuts after 'clear data'
- Also listen to PACKAGE_CHANGED and disable/re-publish shortcuts
properly.

Bug 29355786
Bug 29582255
Bug 29601844

Change-Id: I6c701ce669cf30a227bc2af4aa01de467ef73e3a