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