"Extreme" Lighthouse 1319 patch: Removed class Mail.Mock from Mail

Compared to andreassimon:lighthouse-1319-patch, this one goes quite a step further. The inner class Mail.Mock is extracted to a top-level class LegacyMockMailSystem. But a *constant* named Mail.Mock is left. This way, the invocations of
```Mail.Mock.getLastMessageReceivedBy(String)```
and
```Mail.Mock.reset()```
remain valid.

I must admit that I encountered problems though: I wanted to assure that my modifications don't break any existing apps. Thus, I frequently ran
```
cd framework
ant test
```
During development, test suites
 * just-test-cases,
 * i-am-a-developer, and
 * yabe
failed sometimes. Unfortunately, the test results were NOT RELIABLY REPRODUCIBLE. I would very much appreciate if some of you could help me stabilize this patch :-)