Activity Manager changes the scheduling group of processes.

The algorithm for this is currently very simple: all persistent processes are
always in the normal scheduling group, all other processes are normal if their
oom_adj is as good or better than VISIBLE, otherwise they are in the background
group.

Note that this currently results in a fair number of log messages about not
being able to change the group, since the system process does not have
permission to do so.  Once a kernel fix is in, these will go away and the code
will start working.