Integration test for PLUS_ADDRESS_SETTING removal

The client-side implementation of PLUS_ADDRESS_SETTING has support for
removals (even though there is no plan to implement them on the server
side). This behavior is already covered in unit tests [1]. This CL adds
more test coverage by adding an integration test as well.

The integration test only has access to the setting service (rather than
the model type store). Since non-existent settings behave as if they had
their default value [2], the integration test thus doesn't have a good
way to verify that the setting is actually removed when a tombstone is
received. Instead, it verifies that the setting service indeed returns
the default value. The actual removal is already tested in [1].

[1] https://source.chromium.org/chromium/chromium/src/+/main:components/plus_addresses/settings/plus_address_setting_sync_bridge_unittest.cc;l=191-213;drc=528cbe31ac5fd2202b4b6b1d7bad256f23a02bdd
[2] https://source.chromium.org/chromium/chromium/src/+/main:components/plus_addresses/settings/plus_address_setting_service_impl.h;l=41-42;drc=462e25b394a2d49410368f8bedcdacf32117da6f

Bug: 342089839
Change-Id: I75a0352f2e2e4095345099b85e34deaaa98c617a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5682402
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Cr-Commit-Position: refs/heads/main@{#1324225}