Writing Assistance: Check and consume User Activation

Per spec, user activation is required when the model is downloadable or
downloading during Summarizer / Writer / Rewriter client creation.

It calls `CanCreate*()` to get the model availability status first to
check the download status, before calling `Create*()`. Ideally, this
should be combined as a one mojo round-trip for future improvement.
Since the first call can return unavailable status, now we throw an
error early before calling `Create*()`.

As a result, this fixes missing enterprise check from Create*() mojo
call, which was previously only checked from CanCreate*() mojo call.

This CL also updates permissions policy WPT pattern, to be aligned with Language Detector and Translator API as done in crrev.com/c/6501107 to fix the breakage with the changes introduced in this CL.

Bug: 414811872, 372806447
Change-Id: Ie0ebf35d3e622957139d8a57695e9e4f765430db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6532326
Commit-Queue: Daseul Lee <dslee@chromium.org>
Reviewed-by: Mike Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1461567}