Clean up handling of Exchange setup option

* Use concrete declarations, not the old sample code
* Remove unused "sample" code
* Create ExchangeSender.java
* Strip out the unused (old sample code) parts of ExchangeStore.java
* Unit test to make sure EAS is being offered
* Fix "account leak" in AccountSettingsTests

This was triggered by donut bug # 2092944 but the solution is
different because we support EAS here.
10 files changed