scanner: Abstract out disclaimer prefs logic into separate file

This should be a pure refactor with no behavioural changes. This hides
the implementation detail of what prefs are used to show / hide the
disclaimer.

This currently is a trivial wrapper around getting a boolean from a
pref, but future CLs will extend this further to support for different
entrypoints and reminder disclaimers.

Bug: b:383437797
Change-Id: I6a6a2100edf7a390e8170d83176160ac2cf8fb05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6356186
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Michael Cui <mlcui@google.com>
Cr-Commit-Position: refs/heads/main@{#1433353}