Update power manager to track uid state like netstats.

To follow the correct semantics for when restricts due to
device idle can be applied, power manager need to know about
uid process states like net policy so that it can allow
wake locks from apps that are in the foreground.

Since this is being added to a second place, I reworked things
so that the activity manager now keeps track of per-uid process
states and allows apps to register to listen to those, rather
than having to track lower-level process states and transform
them into an overall uid state.  Both net policy and power
manager use this new facility.

Change-Id: I77359164c40d0f36fe1ef296dd9f9c3062431148