Enhancement: Scoped Storage

The way to store files in the device has changed completely.
Previously, the files were stored in the shared storage. That means that apps that had
access to the shared storage, could read, write or do whatever they wanted with the ownCloud files.

Now, ownCloud files are stored in the Scoped Storage, so they are safer.
Other apps can access ownCloud files using the Documents Provider, which is the native way
to do it, and that means that the ownCloud app has full control of its files.

Furthermore, if the app is removed, the files downloaded to ownCloud are removed too.
So, files are not lost or forgotten in the device after uninstalling the app.

https://github.com/owncloud/android/issues/2877
https://github.com/owncloud/android/pull/3269