ShortcutManager: add remaining APIs.

- Icons are now persisted. (under /data/system_ce, as PNGs)
- the "load icon" APIs in LauncherApps are supported.
- Implement updateShortcuts()

- Addressed all the comments on the previous CL
- @hide the newly added constructor for PersistableBundle

- Enhance incoming shortcut validation
- A lot of internal clean-up.

Bug 27548047

Change-Id: I8e3c1ccd3e0a997a6d271c84d81170f0c022b60e