Add APIs for interacting across users.

- Expose the existing Context.sendBroadcast() as
  Context.sendBroadcastAsUser().
- Add new android:singleUser attribute for services.
- Add new INTERACT_ACROSS_USERS_FULL permission for full
  system-level access to cross-user interface (allows
  sendBroadcastAsUser() to send to any receiver).
- Add new INTERACT_ACROSS_USERS_FULL permission for
  more restricted cross-user interaction: this is required
  for android:singleUser, and allows you to use
  sendBroadcastAsUser() but only to send to your own
  receivers.

Change-Id: I0de88f6718e9505f4de72e3f45d29c0f503b76e9