Sequence DMClient requests

CloudPolicyClient cannot easily support multiple in-flight requests.
Even if it could, concurrent enrollments or policy fetch requests are
difficult to reason about and likely to introduce incorrect behavior.

Create a DMClient wrapper which buffers tasks in a queue, ensuring that
no two operations overlap.

Bug: 403567050
Change-Id: Id1734e177381c1d9e6314b38ba202969dc8ac68a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6354234
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Commit-Queue: Noah Rose Ledesma <noahrose@google.com>
Cr-Commit-Position: refs/heads/main@{#1432981}