Add fallback for Dashclock disk usage
Rethink how valid volumes are determined

Allow system folders to be shown, but only when the menu option is
selected
Fallback to File.getTotalSpace

MountPointHelper.getMountPointDiskUsage fails to work properly
occasionally. In this case, fallback to File.getTotalSpace.
Check DiskUsage less often

Without this, Nexus will not work properly. Not sure why.
Fix Dashclock settings