[Autofill wallet] Implement local changes for metadata sync

This CL implements the following features in the wallet_metadata USS
sync bridge:
 - caching current sync data and filling the cache on start,
 - informing the processor the bridge is ready to sync,
 - reacting to local single modifications of server profiles and cards
   (i.e. to updating the use stats when the address/card is used).

Bug: 853688
Change-Id: I07405e02cdf9cd0eb814179d883594253859845c
Reviewed-on: https://chromium-review.googlesource.com/1151628
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581183}
4 files changed