Fix issue #7311376: Add API to allow apps to know if they are...

...running as the foreground user

Add UserManager.isUserRunning() which is the public version of the
existing method on ActivityManager.

Also add UserManager.isUserRunningOrStopping() since that seems like
it will be useful.

And fix the internal function that returns the array of currently
running users to not include stopped users.

Change-Id: I84672fa8748fc027fd402729586b5603f640e498