Implement issue #10691359: Kill long-running processes

We now have the activity manager kill long-running processes
during idle maintanence.

This involved adding some more information to the activity manager
about the current memory state, so that it could know if it really
should bother killing anything.  While doing this, I also improved
how we determine when memory is getting low by better ignoring cases
where processes are going away for other reasons (such as now idle
maintenance).  We now won't raise our memory state if either a process
is going away because we wanted it gone for another reason or the
total number of processes is not decreasing.

The idle maintanence killing also uses new per-process information
about whether the process has ever gone into the cached state since
the last idle maintenance, and the initial pss and current pss size
over its run time.

Change-Id: Iceaa7ffb2ad2015c33a64133a72a272b56dbad53