Reuse mem cgroups for tracking forked PIDs.

It turns out we were using the CPU accounting
cgroups for keeping track of processes that were
forked by an app without the framework's knowledge,
so we could kill all of them reliably (see b/15313911
for context).

Since we want to use memory cgroups for other purposes,
we might as well use memory cgroups for tracking forked
PIDs if they're enabled. This also gets us automatic cleanup
of empty mem cgroups.

Also, removed old mem cgroup mount point that is no
longer used, as well as cgroup release agent code that
we're not using.

Change-Id: I69d5cc31c162ffa49ef6945755f41381e306cc8b