Cache for Controller.getServiceForAccount.

Change-Id: I607c437d6e0781ae8305df22c13a0e3a003c623f
Cleaning up MessageList

- Removed Handler.
- Refactored FindMailboxTask so it only does DB access on a worker thread.
  (Moved the actual task out of doInBackground)
- Removed unused imports, which I forgot when I extracted the adapter.

Change-Id: Ib76ce2ab7901dd39d2ed51d8a61d7be9df55b337
Merge "Cache for Controller.getServiceForAccount."
Fix the cache in Controller.

Turned out we can't reuse EmailServiceProxy, so only cache the account type.

Change-Id: I5499c2440082580d6f0739ae85b888f1afb65ba8
Fix typo on exchange server setting screen.

Bug 2742113

Change-Id: I43bd3d6328745169bacb1fd750cce0a08dc18742
Merge commit 'afa85bb10269f4ecad4190d91ffe058a147c25ee' into oxygen_1006

* commit 'afa85bb10269f4ecad4190d91ffe058a147c25ee':
  Fix typo on exchange server setting screen.
  Fix the cache in Controller.
  Cleaning up MessageList
  Cache for Controller.getServiceForAccount.