Use Storage Access Framework in demo app to access local files.

### Use case description
Starting with Android Q, access to random local files is forbidden unless the Storage Access Framework is used (see e.g. #6039). To simplify debugging of local files with our demo app, we should add an option to open a file this way. On the other hand, we no longer need to request permission to read files.

See https://developer.android.com/preview/privacy/scoped-storage and https://developer.android.com/guide/topics/providers/document-provider