[CrOS Network] Fix flash of content when network config dialog loads.

The flash was caused by the fact that separate parts of the
configuration dialog load at different times. To fix this issue, a
spinner is shown in the dialog while the dialog loads its fields. Once
the fields are all loaded, the spinner is removed and the rest of the UI
is shown.

(cherry picked from commit 4b315cc333b822cc66de685bac74574219e8291d)

Bug: 905231
Change-Id: I1ee4306cce945f71ff5ce6c71d3518c4bdd2c70a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649222
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#668070}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1656378
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/branch-heads/3809@{#264}
Cr-Branched-From: d82dec1a818f378c464ba307ddd9c92133eac355-refs/heads/master@{#665002}

• ui/webui/resources/cr_components/chromeos/network/network_config.html[diff]
• ui/webui/resources/cr_components/chromeos/network/network_config.js[diff]