Improve handling of low memory.

Now classify background processes into a set of bins of how much
memory they should try to clear.  The last bin also involves
destroying all activities in that process.

Removed the old code for the simulator that is no longer needed
(yay).  The debugging features it had are now integrated into the
regular oom adj code.

Small fixes to load average service.

Change-Id: Ic8df401714b188c73b50dbc8f8e6345b58f1f3a0