Favor Context over Application

We passed `Application` references in many places where a `Context` is sufficient. This first commit fixes that.

Using `K9.app` introduces unnecessary coupling and makes testing harder. The second commit replaces occurrences of `K9.app` with a `Context` reference where easily possible.

PS: Don't use `K9.app`!
Hmm, not sure whether that's a good idea, both from the code clarity and the 'effort needed for conversion' point of view. Just because IDEs do highlight members and static separately, that doesn't mean all viewers do that - the Github client for Android I'm using to write this doesn't, just to name one; so looking at the code just for review purposed gets harder for very little perceivable gain (if any).

But you're the main contributor, so it's your call, of course.