Don't back-off ALREADY_IN_PROGRESS syncs

BUG: 16219182
The issue is that some adapters get into a bad state where
SyncManager assumes them complete yet they are still going. This
result in ALREADY_IN_PROGRESS coming back from the adapter, the
default retry-time for which is about 10s.
Subject ALREADY_IN_PROGRESS to exponential back-off to avoid waking
up SM every ~10 seconds

Change-Id: I6cef787366436c678bac762ec6daf6212f04badc