Implement loading data from sync in AccountSettingSyncBridge

Adds logic initialising the ModelTypeStore, loading existing metadata
and calling ModelReadyToSync().

Also defines an in-memory cache for all ACCOUNT_SETTING specifics. This
will allow exposing setting values to AccountSettingService
synchronously.

See go/sync-client-account-setting.

Bug: 441735283
Change-Id: I2384ecbce82e292bd3b30165c521a5f7d40402fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6994902
Commit-Queue: Jihad Hanna <jihadghanna@google.com>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Cr-Commit-Position: refs/heads/main@{#1525459}