[Sync] Replay incoming invalidations on adding listeners

This CL introduces collecting incoming FCM messages in a limited buffer
to replay them whenever a new listener is added. It's useful in several
cases. On Android it will replay incoming invalidations which were
received in background. It's also useful for debugging purposes to show
latest incoming messages on sync-internals page.

An integration test will be added in a follow-up CL.

Bug: 1329060
Change-Id: I9dd6fbc4a01d2cdb29991a125298f2abd4e731de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3666716
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Rushan Suleymanov <rushans@google.com>
Cr-Commit-Position: refs/heads/main@{#1007334}