Enhancement: Move to AndroidX Preference and new structure for settings

Settings have been updated to use the current Android's recommendation, AndroidX framework.
In addition, they have been reorganized into subsections for a better understanding and
navigation structure. Also, new features have been added: now, source path and behaviour
in auto uploads can be chosen differently for pictures and videos.

https://github.com/owncloud/android/issues/2867 https://github.com/owncloud/android/pull/3143
