Email split, part trois: AccountService

* Create AccountService.aidl and AccountServiceProxy in emailcommon
* Implement AccountService in email
* Use AccountServiceProxy in Exchange for account reconciliation,
  notifications, etc.
* Move sync window constants into emailcommon
* Split attachment provider utilities and constants into emailcommon

Bug: 3442973
Change-Id: I89dce28b799b193243c07774dab65d830ae62775
7 files changed