Make per-application memcg hierarchy configurable via a property

Introduce ro.config.per_app_memcg Android property to enable/disable
per-application memcg hierarchy.
On low-ram devices (ro.config.low_ram=true) ro.config.per_app_memcg
defaults to true, otherwise it defaults to false.

Bug: 80201565
Test: set ro.config.per_app_memcg=true and verify /proc/<app_pid>/cgroup
membership (memory:/apps/uid_xxx/pid_xxx)
Test: set ro.config.per_app_memcg=false and verify /proc/<app_pid>/cgroup
membership (memory:/)
Change-Id: Iabd3ef6c9c16c774b9da959a7f43b5f66df120a3
Signed-off-by: Suren Baghdasaryan <surenb@google.com>