Implement prefetch of IMAP/EAS attachments

* Load attachments in the background for IMAP/EAS messages
* Download an attachment from account X if:
  1) 25% of total storage free
  2) Attachments for X use < 1/N of 25% of total storage, where N is
     the number of AccountManager accounts
* Add accountKey to Attachment table for performance

Change-Id: I913aa710f34f48fcc4210ddf77393ab38323fe59
2 files changed