ANDROID: GKI: Add cgroup ABI padding

To facilitate potential future backports and fixes.

Specifically, this padding is meant to accommodate new fields necessary
to fix incorrect /proc/cgroups accounting as reported at [1]. Reserve
space for an array of atomic_t for each subsystem to track css counts.
Additionaly reserve space for a pointer in struct cgroup in case we must
implement hierarchical accounting instead of root-only accounting.

sizeof(atomic_t) < sizeof(u64), so a byte array is used to reserve this
space to avoid memory waste if we were to try to stuff atomic_ts into an
array of u64s.

[1] https://lore.kernel.org/all/20240528163713.2024887-1-tjmercier@google.com/



Bug: 293511579
Change-Id: I498b4a3a2736f2d66ecd70d63577e756a8c69db0
Signed-off-by: T.J. Mercier <tjmercier@google.com>