Fix update prompts on disjoint entities of the same type

This CL updates the entity-merging logic to ensure that two entities
first match on a set of predefines attributes before checking if they
are mergeable. Not matching would mean they are not the same entity and
hence no update prompt should be shown but instead a save prompt should.

Fixed: 403187605
Change-Id: I047a50768d221aebcd75a47f5ec660f438c8e79d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6361094
Commit-Queue: Jihad Hanna <jihadghanna@google.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1433510}