Flesh out multi-user in am commands.

Now we default to the current user instead of user 0 for most commands
(except where we can do the command for all users).

Many more commands take a user argument: force-stop, kill, profile,
dumpheap.

Improved help text.

Change-Id: I719a13b4d31b668f57ca21e51d7043ac3e0d4e1b