releasetools: Fix the bug when deleting files.

For file-based OTAs, we used to remove unneeded files in ascending
order, which failed to delete non-empty directories. Reverse the order
to fix the issue.

For example, now we have the following in our generated script:
  delete("/system/app/Calculator/arm/Calculator.odex",
         "/system/app/Calculator/arm/",
         "/system/app/Calculator/Calculator.apk",
         "/system/app/Calculator/");

Bug: 22960996
Change-Id: I0d36d29b7862fb53bf55bf5685a990180f9c0b3b

tools/releasetools/ota_from_target_files

@@ -1101,11 +1101,13 @@ class FileDifference:
       script.FileCheck(tf.name, tf.sha1)
 
   def RemoveUnneededFiles(self, script, extras=()):
-    script.DeleteFiles(["/"+i[0] for i in self.verbatim_targets] +
-                       ["/"+i for i in sorted(self.source_data)
-                              if i not in self.target_data and
-                              i not in self.renames] +
-                       list(extras))
+    file_list = ["/" + i[0] for i in self.verbatim_targets]
+    file_list += ["/" + i for i in self.source_data
+                  if i not in self.target_data and i not in self.renames]
+    file_list += list(extras)
+    # Sort the list in descending order, which removes all the files first
+    # before attempting to remove the folder. (Bug: 22960996)
+    script.DeleteFiles(sorted(file_list, reverse=True))
 
   def TotalPatchSize(self):
     return sum(i[1].size for i in self.patch_list)