Use ACTION_OPEN_DOCUMENT for Android >= KitKat. Fixes #926.

We have to do this, since with the new Storage Access Framework, otherwise we can open the Uri only *once*. This would work well unless someone saves a draft and goes back to the conversation - then the Uri is opened again without the required permissions. See

https://developer.android.com/guide/topics/providers/document-provider.html#client

for details.