K9 mail code as plugin

This was the goal of previous refactoring efforts (mainly #516). With all the mail handling code now completely isolated from the app code it is feasible to move it into a separate module, which will ensure the strict separation in the future.

As a nice side-effect it will also make it possible for other apps to easily pull in K9's mail handling code.

Some outstanding issues:

 * `TracingPowerManager` should probably not be in the mail package. I couldn't find a good way to isolate this.
 * Probably does not build with ant as-is
 * Gradle build files need to be streamlined