a11y: Create a source set for accessibility_state_utils

Create a source set, "utils", to build accessibility_state_utils.h/cc.
It makes it easy for other modules to depend on it and use functions
defined there.

Note that this CL is a pure refactoring and does not change any
behaviors. A follow-up CL will use and depend on this new source set.

AX-Relnotes: n/a.
Bug: b:289010799
Change-Id: Id0a3c87e28d997fd2862546d00f547cd707439cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5466994
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Commit-Queue: Kyungjun Lee <kyungjunlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1294405}