Introduce CachedDevicePolicyUpdater

as a testing utility to update device policy in
FakeSessionManagerClient. There are similar utilities,
e.g. DevicePolicyCrosTestHelper, ScopedDevicePolicyUpdate,
DeviceStateMixin etc. However, these do not take care of each other,
and mix of these uses occasionally causes unexpected behavior.
The new testing utility is intended to replace these use cases, too,
but will be done separately.

BUG=414927997
TEST=Tryjob

Change-Id: I5ef86729bdf481e3a449721193da61512a3ee59d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6615389
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Igor <igorcov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1469243}