[App Update] Rename feature flag to predictable app updating

The app updating design has changed directions from using an update
token from being parsed to using changes in icon urls for making them
predictable. As such, rename the feature flag to make the intentions of
the flag clearer.

Bug: 403252297
Change-Id: I0a61c9522b71a89f28d7911c9a9e9d4fb7413a38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6715817
Reviewed-by: May Siem <msiem@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Dibyajyoti Pal <dibyapal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1484343}