updater: create the network fetcher only if needed in the DMFetch

For the vast majority of users, a policy fetch is not needed. However,
the execution path creates a DMFetch object, which instantiates a
network fetcher. This is an undesired side effect, because this network
fetcher is not used.

Fixed: 339302599
Change-Id: I46e92b613a188fcecfd97d0b61f2be82638798b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5805664
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Auto-Submit: Sorin Jianu <sorin@chromium.org>
Commit-Queue: Xiaoling Bao <xiaolingbao@chromium.org>
Reviewed-by: Xiaoling Bao <xiaolingbao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1345149}