Enhancement: Allow access from document provider preference

Previously, files of ownCloud accounts couldn't be accessed via documents provider
when there was a lock set in the app. Now, a new preference has been
added to allow/disallow the access, so users have more control over their files.

https://github.com/owncloud/android/issues/3379
https://github.com/owncloud/android/issues/3520
https://github.com/owncloud/android/pull/3384
https://github.com/owncloud/android/pull/3538