Choice between All Files and read-only external storage in SDK >=30

- Allow users to choose between MANAGE_EXTERNAL_STORAGE (full access) and READ_EXTERNAL_STORAGE (media read-only) in sdk >=30, with a dialog
- If All Files is not available (activity to manage not present, or permission not in manifest), request READ_EXTERNAL_STORAGE instead
- Misc improvements to permission request in UploadFilesActivity and SyncedFoldersActivity

Signed-off-by: Álvaro Brey Vilas <alvaro.brey@nextcloud.com>