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.