Rework the LRU list for hidden and empty processes.

This is intended to solve a problem on devices with more memory
where we can fill up that memory with processes that contain activities
(hidden processes), leaving no room for empty processes.  Thus if a
process is receiving broadcasts regularly, or starting and stopping
a service, or such, we will continually create its process only to
have it immediately killed when done.

There is certainly some tuning that should be done on this as we
look at the actually behavior.  The implementation here puts all of
the hidden and empty processes into one list, trying to make some
preferences for the very most recently used activity's processes to
stay at the top and not get pushed out by other processes being
started in the background.