[AF Sync] React to remote changes in wallet metadata sync bridge

This CL implements reacting to remote changes in the new wallet metadata
sync bridge.

Unit-tests are not part of the CL as I want to first get the bridge into
state to pass integration tests (fix all issues on the way) and then
only write unit-tests to test the final behavior.

Bug: 853688
Change-Id: Ia9bd8157a48771a0813b326d5b82d98c3144841e
Reviewed-on: https://chromium-review.googlesource.com/c/1406715
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622305}
6 files changed