Limit user mailboxes for EAS (to 1000, for now)

* Sending too many mailboxes to EmailProvider in a single batch can
  lead to a binder exception, so we limit them
* Move all provider updates into the commit() method

Bug: 3325331
Bug: 3314442

Change-Id: I1bd8d19a96f642b6e49e2f47a5ddcebaa55c0ec0
1 file changed