[CrOS Cellular] Fix APN row item connected state not refreshing

Fix APN list item "Connected" sublabel not updating state correctly if
the list item's APN changes or its connection state changes. Previously,
once the "Connected" label was shown for a row item, it would never be
removed, or vice versa. This is because only the index was used in the
Polymer binding. Updated to refresh if the managed properties changes.

Bug: b:162365553
Test: DUT and *ApnList*
Change-Id: I20c60f4ed0412a0122e21a260e39b0faf860e2f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4400394
Auto-Submit: Gordon Seto <gordonseto@google.com>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Commit-Queue: Gordon Seto <gordonseto@google.com>
Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1126330}