Add support for launcher shortcuts.

Add support for launcher shortcuts.

- This CL has no UI but provides the necessary backing for one.
- Adds new item type: ITEM_TYPE_DEEP_SHORTCUT, to distinguish from
  ITEM_TYPE_SHORTCUT. We can reconsider these names.
- Adds ShortcutCache, using LruCache for up to 30 dynamic shortcuts
  (pinned shortcuts are always cached in a HashMap).
- DeepShortcutManager queries for shortcuts and other things like
  pin them. In a future CL it will use the cache, but for now it
  simply makes an RPC for all queries.
- LauncherModel maintains counts for pinned shortcuts, pinning and
  unpinning when counts reach 1 or 0, respectively.
- LauncherModel maintains a map of components to lists of shortcut ids,
  which Launcher gets a copy of after it is changed in the background.
  This will allow us to know how many shortcuts an app has immediately,
  and query for details as the UI is animating.

Change-Id: Ic526f374dd10d72a261bae67f07f098fca8d8bca