Various clean-ups in SyncInternalsMessageHandlerTest

Test-only change.
1. Modernize use of base::Value APIs.
2. Override UserEventService earlier, in GetTestingFactories().
3. Use a MockSyncInvalidationsService instead of the real one.
4. Remove last_delegate_sync_service() which has little use.
5. about_information() -> kAboutInformation
6. Reduce number of APIs that TestableSyncInternalsMessageHandler
   needs to publicize.
7. Remove the SendAboutInfoSyncDisabled test, which verifies an
   impossible scenario - OnStateChanged() notification when
   SyncService is null.
8. Make ConstructFakeAboutInformation() private.

Bug: 360321896
Change-Id: Ibe00dcb3091fab677702e617e3be1455b3a3d7e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5797069
Reviewed-by: Mahmoud Rashad <mmrashad@google.com>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1344098}