Spanify dbus::MessageReader::PopArrayOf...

The `MessageReader::PopArrayOf...` methods were returning raw pointers,
which were being accessed unsafely. They have been updated to return a
`base::span` instead of a raw pointer and size. This makes the API
safer, readable and prevents unsafe buffer access.

All the call sites have been updated, and simplified.

Generated with gemini-cli, following by multiple AI assisted iterations.

Bug: 435317390
Bug: 435682511
Fixed: 435682511
Change-Id: Icf6332cf7453018da481297836f5cf98ab3d81f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6811455
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1501289}
NOKEYCHECK=True
GitOrigin-RevId: b77eea0871022fa9a537892679ebfcc09ca34104