Remove SortedStringKeys

We have SortedKeys instead now.

Test: Presubmits
Change-Id: I27b38c63cdd770d6bef70e08ae38fdddc066c0f8
diff --git a/filesystem/android_device.go b/filesystem/android_device.go
index 47c4e3d..4ee5681 100644
--- a/filesystem/android_device.go
+++ b/filesystem/android_device.go
@@ -351,7 +351,7 @@
 	if a.partitionProps.Super_partition_name != nil {
 		superPartition := ctx.GetDirectDepProxyWithTag(*a.partitionProps.Super_partition_name, superPartitionDepTag)
 		if info, ok := android.OtherModuleProvider(ctx, superPartition, SuperImageProvider); ok {
-			for _, partition := range android.SortedStringKeys(info.SubImageInfo) {
+			for _, partition := range android.SortedKeys(info.SubImageInfo) {
 				filesystemsToCopy = append(
 					filesystemsToCopy,
 					targetFilesystemZipCopy{info.SubImageInfo[partition], strings.ToUpper(partition)},