libprocessgroup: memcg default version 1 -> 2

The default memcg version is 1 in cgroups.json. Update the default to
version 2. Vendor overrides of cgroups.json [1] (to use v1 with the v2
default here) remain supported for kernels which still support memcg v1,
but core features of v1 have already been deprecated and removed from
recent kernels, and it is expected v1 will eventually be removed from
the kernel entirely. So v1 users should migrate to v2 as soon as
possible.

This change activates memcg only in the root, but we may update this to
enable per-application memcgs by default in the future. Vendors may
override this default with their own cgroups.json to get per-application
memcgs by adjusting the value of MaxActivationDepth.

[1] https://source.android.com/docs/core/perf/cgroups#per-api-level-task-profiles

Test: (cuttlefish) adb shell grep memory /sys/fs/cgroup/cgroup.controllers
Bug: 377579705
Change-Id: I5b985fb4348b92d49022760331c0adf113bcdf04