libprocessgroup: memcg v2 default MaxActivationDepth = 3

Enable per-PID memcgs by default.

Vendor overrides to alternate MaxActivationDepths are supported in Android 16. Overrides to memcg v1 are also supported in Android 16. [1]

In Android 17 these overrides are not likely to be supported.

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

Bug: 386797284
Change-Id: Ia2107ac2244dcd9d05c918fe964529d0f83a13cf