Replace SortedStringKeys with SortedKeys

Now that we have generics.

Bug: 193460475
Test: presubmits
Change-Id: I1594fd8feb505175d5c09c03ef397e5ffd5b09cb
diff --git a/java/classpath_fragment.go b/java/classpath_fragment.go
index 259e977..cf81ddb 100644
--- a/java/classpath_fragment.go
+++ b/java/classpath_fragment.go
@@ -111,7 +111,7 @@
 			ctx.PropertyErrorf("contents", "%v is not a ModuleWithStem", name)
 		}
 	}
-	return android.SortedStringKeys(set)
+	return android.SortedKeys(set)
 }
 
 // Converts android.ConfiguredJarList into a list of classpathJars for each given classpathType.