Periodic sync management was wrong in at least two ways:
1) Periodic syncs were scheduled as a side effect of
performing a sync, so changing the sync interval setting
would not do what you expect.
2) Performing a manual mailbox sync would schedule a
periodic sync on that mailbox.
This change removes the side effect from sync, and instead
actually responds to updates to the sync interval by
removing the extra syncs and updating just the account sync.
Bug: 8666490
Change-Id: I3f4a3fb7044c408578f0dc75ccb6c5bb176bfc0b
e669f28b08