Add intent filter to main activity to provide default email application setting.

Adds intent filter [`CATEGORY_APP_EMAIL`] in order to provide the app's activity as possible default email application. This must be interpreted by Android and [set by the user outside of the application][1].

The new intent-filter for `APP_EMAIL` was added to activity `MessageList`.

[`CATEGORY_APP_EMAIL`]: https://developer.android.com/reference/android/content/Intent.html#CATEGORY_APP_EMAIL
[1]: https://github.com/k9mail/k-9/issues/3231#issuecomment-515759615