Add Document/Storage API support

This will add support for the storage API introduced with Android 4.4 in encryption and decryption dialogs.

Although the current state works, we should consider some UI changes: The editable text field that showed the whole file path before, shows only the file name when using storage API. This is because files in storage API do not necessarily have a path and the internal URI is not intended to be human readable. Editing the text field does not make sense anymore - maybe the EditText should be replaced by a TextView for 4.4 ?