Start implementing concept of "running" users.

The activity manager now keeps track of which users are running.

Initially, only user 0 is running.

When you switch to another user, that user is started so it is
running.  It is only at this point that BOOT_COMPLETED is sent
for that user and it is allowed to execute anything.

You can stop any user except user 0, which brings it back to the
same state as when you first boot the device.  This is also used
to be able to more cleaning delete a user, by first stopping it
before removing its data.

There is a new broadcast ACTION_USER_STOPPED sent when a user is
stopped; system services need to handle this like they currently
handle ACTION_PACKAGE_RESTARTED when individual packages are
restarted.

Change-Id: I89adbd7cbaf4a0bb72ea201385f93477f40a4119