fake_dmserver: support string-list policies

Every other policy type is supported, so bring it to parity.

Change-Id: I9c4608e66244a993a71b1a98ad2e21496a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8521389
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Igor Ruvinov <igorruvinov@chromium.org>
Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1715883}
NOKEYCHECK=True
GitOrigin-RevId: b04e12eb82ad31ddba9ef2526560e525410848bf