[PA] Parametrize Empty Slot Span Ring Buffer size

This helps to fine-tune the constant through an experiment.
Ring size configuration is now handled outside PartitionAlloc, because
concept of "foreground" / "background" is Chrome-specific and should be
taken care of at `//base` layer.

Bug: 329199197
Change-Id: I4fadbd3fc0cae83a03e1fb5a1b2a97ba389de4af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7310265
Owners-Override: Mikihito Matsuura <mikt@google.com>
Reviewed-by: Kalvin Lee <kdlee@chromium.org>
Commit-Queue: Mikihito Matsuura <mikt@google.com>
Cr-Commit-Position: refs/heads/main@{#1564225}
NOKEYCHECK=True
GitOrigin-RevId: 4cc0aec02067ba8ce5801f270ba68c07c44348fa