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`!
New code style. See the recent post on the k-9-dev mailing list.