libprocessgroup: Add MaxActivationDepth

Cgroup v2 controllers can be enabled in a subtree of the shared
hierarchy. That allows users to limit the number of cgroups with a
controller enabled to less than the total number of cgroups. [1]

There are costs for each cgroup. Kernel memory is used for each cgroup,
plus additional memory for each active controller in each cgroup. Some
kernel operations scale with the number of cgroups (with a given
controller enabled), so it can be desirable to minimize the number of
cgroups with that controller enabled.

This change allows each v2 controller configuration to specify a maximum
activation depth, past which the controller will not be activated deeper
in the Android cgroup v2 hierarchy. The hierarchy root is defined as
depth 0. MaxActivationDepth is the field name for this purpose for
controllers in the Controllers array under Cgroups2 in cgroups.json.

Here are two examples:

"MaxActivationDepth": 1
This will activate the controller in every per-application cgroup, but
not in the per-process cgroups below.

/sys/fs/cgroup                depth=0  active=true  (controller listed in cgroup.subtree_control)
/sys/fs/cgroup/uid_0          depth=1  active=true  (controller NOT listed in cgroup.subtree_control)
/sys/fs/cgroup/uid_0/pid_100  depth=2  active=false (controller NOT listed in cgroup.subtree_control)

This can also be used with
PRODUCT_CGROUP_V2_SYS_APP_ISOLATION_ENABLED := true.

"MaxActivationDepth": 1
This will activate the controller only at the app / system level, but
not in per-application cgroups below. This results in a total of only
3 cgroups with the controller enabled (root, apps, system).

/sys/fs/cgroup                         depth=0  active=true  (controller listed in cgroup.subtree_control)
/sys/fs/cgroup/apps                    depth=1  active=true  (controller NOT listed in cgroup.subtree_control)
/sys/fs/cgroup/apps/uid_10000          depth=2  active=false (controller NOT listed in cgroup.subtree_control)
/sys/fs/cgroup/apps/uid_10000/pid_100  depth=3  active=false (controller NOT listed in cgroup.subtree_control)

[1] https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#enabling-and-disabling

Bug: 346584259
Test: Cuttlefish with memcg v2
Change-Id: I62109ea935261c51fc30b2054c4d28d0360f7985