Don't cache ImapFolders

* ImapFolder is currently very unsafe for use by multiple threads,
  causing, among other things, the referenced bug
* Since ImapFolder is very lightweight, there's no particularly good
  reason to be caching them anyway
* Rename isOpenForTest to isOpen

Bug: 4972084
Change-Id: I2bf17b9cfc8549a222e991f3e59abfd00a4d3afd
7 files changed