[CrOS Cellular] Fix sending pre-revamp APNs when network migrated

Update ApnMigrator to send the revamp APN list to Shill if the ApnRevamp
flag is on and the network has already been migrated, rather than the
pre-revamp APN list as was done previously. This would cause Shill to
incorrectly use the legacy APN list when the flag is enabled.

Also reorganize some logic to separate out the legacy and revamp cases
more, and add more documentation.

Bug: b:162365553
Test: ApnMigratorTest*
Change-Id: I87e8ab293be8b99d19d1a4147ef3c033439e8446
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4143863
Commit-Queue: Gordon Seto <gordonseto@google.com>
Reviewed-by: Chad Duffin <chadduffin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1094023}