libprocessgroup: Look for memcg v2 MaxActivationDepth override

Init may write a file to /metadata containing a server override for
memcg v2 MaxActivationDepth for devices in an experiment population.
Use this value instead of the value from cgroups.json if the file
exists.

This will be reverted after the memcg v2 experiment is complete, and
before the final 25Q2 TOT snap. The final MaxActivationDepth value will
be commited to cgroups.json on AOSP. go/android-memcgv2-exp

Bug: 384577842
Test: setprop persist.device_config.mglru_native.lru_gen_config all
Test: grep memory /sys/fs/cgroup/*/cgroup.subtree_control
Test: reboot, then grep memory /sys/fs/cgroup/*/cgroup.subtree_control (again)
Test: Also tested with corp credentials and server flags in teamfood/droidfood.
Ignore-AOSP-First: 25Q2 Beta Experiment only go/android-memcgv2-exp
Change-Id: I960fe503bebc020867bb0c71be4e1bac282a1c0d