Change: Upgrade min SDK to Android 6 (API 23)

The minimum SDK has been updated to API 23, which means that the minimum
version of Android we'll support from now on is Android 6 Marshmallow.

https://github.com/owncloud/android/issues/3245
https://github.com/owncloud/android/pull/4036
https://github.com/owncloud/android-library/pull/566
