ANDROID: make per-cgroup PSI tracking configurable

With cgroup v2 hierarchy enabled PSI accounts stalls for each cgroup
separately and aggregates at each level of the hierarchy. That causes
additional overhead since psi_avgs_work would be called for each
cgroup in the hierarchy.
In Android we use PSI only at the system level, therefore this overhead
can be avoided.
Introduce CONFIG_PSI_PER_CGROUP_ACCT that controls per-cgroup PSI
tracking and is disabled by default.

Bug: 178872719
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I70a418aba76b46a27eb9e66080434aa870496384