Add registering for explicit users.

New API to register as an explicit user, which allows you to
also select ALL to see broadcasts for all users.

New BroadcastReceiver API to find out which user the broadcast
was sent to.

Use this in app widget service to handle per-user package broadcasts
and boot completed broadcasts correctly.

Change-Id: Ibbe28993bd4aa93900c79e412026c27863019eb8