Modify how the background process LRU list is handled.

A long time ago, we had a concept of an "empty" process -- this was
a process that didn't have any interesting components in it, which
would be placed below everything else in the LRU list.

Empty processes didn't work out well, because you could get into
bad situations where you have filled your LRU list with things that
have hidden activities, pushing empty processes to the bottom and
being immediately killed as soon as they go into the list.  So this
was removed.

This change brings the concept back, but in a slightly different
form, to address a more specific problem: for people who are switching
between N different applications, we would like to try to keep those
activities available in RAM in a consistent manner.  Currently the
previous activities would be killed often quickly and suprisingly,
even on devices with lots of RAM.  This is for two reasons:

(1) As you sit in one application, other things going on in the
background will go to the top of the LRU list, pushing down the
previous apps you have visited, even though you aren't aware at all
of these other things executing.
(2) There is a hard limit on the number of background processes
(currently 16) after which they are killed regardless of the amount
of available RAM.  This is desireable because if there is lots of
RAM we can end up with tons and tons of processes sitting around,
not really serving any purpose, but using up resources.

To improve the situation, we have again a concept of "empty" processes
but now it means one with no activities.  Processes that aren't empty
but in the background list are called hidden.  We maintain these as
two parallel lists, each getting half of the process limit: so with
a 16 process limit, you can have at most 8 empty and 8 hidden processes.

This allows us to consistently keep up to 8 recent applications around
for fast app switching; we will also keep around 8 other processes to
make it more efficient for background work to execute again if it needs
to.

Change-Id: Iee06e45efc20787da6a1e50020e5421c28204bd7