SftpPlugin: use MANAGE_EXTERNAL_STORAGE instead of SAF in Android 11+
This new permission gives us full access to the filesystem without having to pick specific locations to share which gave problems due to most locations not being selectable in Android 12+.
This MR keeps the SAF-based method for Android 10 and older but uses MANAGE_EXTERNAL_STORAGE in 11 and later.
Edited by Albert Vaca Cintora