[CrOS Cellular] Replace ApnPolicies flag with AllowApnModificationPolicy

For codepaths relevant to allow APN modification policy only, replace
ApnPolicies gate with AllowApnModificationPolicy.

Bug: b/162365553
Change-Id: I9aab948a050f00b9e2b94be94b1f394111e8fda7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5616172


Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Gordon Seto <gordonseto@google.com>
Cr-Commit-Position: refs/heads/main@{#1313030}