Enable apps to exclude their own roots from the DocumentsUI roots list.

- Add DocumentsContract.EXTRA_EXCLUDE_SELF (boolean extra)
- Add code to DocumentsActivity to cache the providers of the calling
  app if EXTRA_EXCLUDE_SELF is set on incoming Intents.
- Add code to RootsCache to exclude roots from the calling app.
- Add code to allow only system apps to use EXTRA_PACKAGE_NAME.

Change-Id: Ia7cc2a1a297676c7b26f6e583042a4607d8c9a4e