extensions: Move permissions_helper.h/cc into permissions API directory

These permissions helpers are only used by the API and its event router.
They are really internal implementation code for the API (and they
depend on the API).

In the spirit of non-API code not depending on API code, move the
helpers into the API directory. Rename to permissions_api_helper.h/cc
to emphasize they are for the API. (The code already uses the namespace
"permissions_api_helper" so this matches nicely.)

This eliminates the last #include of //c/b/extensions/api/permissions
code coming from outside that directory, apart from tests.

Bug: 395936651
Change-Id: I19475caf4cee5d04dd6efb704fc22cfa91ebe44e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6270382
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1420781}