[CrOS Cellular] Enhance ApnMigratorTest.ApnRevampFlagDisabled unit test

Add checks to make sure that migrated networks will only be cleared when
the APN revamp flag is disabled. The migrator should not check the
custom APN list for them in this case.

Bug: b:162365553
Test: chromeos_unittests --gtest_filter=ApnMigratorTest.*
Change-Id: Ia53c13bcea0f729ba3e6c482d8d9d42ea7ba0c6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4085052
Commit-Queue: Emmanuel Arias Soto <eariassoto@google.com>
Reviewed-by: Chad Duffin <chadduffin@chromium.org>
Reviewed-by: Gordon Seto <gordonseto@google.com>
Cr-Commit-Position: refs/heads/main@{#1081782}