[PA] Make functions related to allocation start/size general purpose

These functions were originally created for BRP, but there was close to none BRP-specific code, so they were trivial to make general purpose.

This prepares to adding an "in range" check at base::span<> construction time.

Bug: 1523232
Change-Id: Ibf11849978bf78d9ea803d71a8ce39954f6b8da8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5247740
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1254278}