Add tracking of uid process states in battery stats.

We now keep track of how long each uid had processes in
various states: foreground, active, running.  This is based
on a collapse of the various activity manager process states
into these three bins.

You'll see these in a checkin like this:

8,10013,l,st,61504,61504,83109

Also fix issue #16021555: App showing up as on "top" even
when the screen is off.  This is "fixed" by just saying we
always report the current app at the top of the activity stack,
regardless of the state of the screen.

Change-Id: I1204904225101243eb00b43425d9806bffdd2ab9