Clean up allocation accounting

Add a "RACING_DCHECK" for counter comparisons that are read with
memory_order_relaxed, and thus might legitimately be inconsistent.
This is a hack, but so long as it's only used for DCHECKs, it seems
better than other available options.

Several counters used inconsistent memory_order specifications.
Generally move those to memory_order_relaxed for informational counters
where the value does not affect code execution in a way that matters
for correctness, and where performance might be an issue.

In cases in which performance is clearly not an issue, just remove
the memory_order specifications, thus no longer erroneously implying
that we've actually thought about ordering.

Improve comments in a few places where I found them confusing.

Bug: 79921586

Test: build and boot AOSP.
Change-Id: I8d0115817a9ff47708ed5e92e8c9caca7e73f899