[AF Wallet] Notify remote metadata changes

This CL introduces notification of remote changes for wallet metadata
sync. This is vital for two reasons:
 - PersonalDataManager loads up to date data as soon as it gets changed;
 - It allows to write integration tests for wallet metadata that only
   observe the PDM (and thus test as end-to-end as it gets).

The approach to block self-notification is a bit hacky; it will get
removed in the new USS bridge as this will not listen to MultipleChanged
any more.

Bug: 894001
Change-Id: I1bcdd4b80187c2980c4343bfd37584ad64d93d01
Reviewed-on: https://chromium-review.googlesource.com/c/1309744
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607215}
3 files changed