Improve multi-user broadcasts.

You can now use ALL and CURRENT when sending broadcasts, to specify
where the broadcast goes.

Sticky broadcasts are now correctly separated per user, and registered
receivers are filtered based on the requested target user.

New Context APIs for more kinds of sending broadcasts as users.

Updating a bunch of system code that sends broadcasts to explicitly
specify which user the broadcast goes to.

Made a single version of the code for interpreting the requested
target user ID that all entries to activity manager (start activity,
send broadcast, start service) use.

Change-Id: Ie29f02dd5242ef8c8fa56c54593a315cd2574e1c