Make sure that account mailbox is always "syncable"

* The test isMailboxSyncable in ExchangeService lumps the account
  mailbox in with regular email mailboxes; this means that the
  account mailbox won't sync if "Sync Email" isn't checked in the
* Fix this by always allowing the account mailbox to sync
* Also, make sure we're upgrading account flags properly for ICS

Bug: 5582884
Bug: 5532630
Bug: 5577486

Change-Id: Iee93eab31adf7a844260a0abc490280b08977a61
2 files changed