I would recommend Respresso which is an online tool capable to create all the required sizes (from xxxhdpi to mdpi) for Android (and for iOS also).
You have to upload the images (in batch) then sync the resized/converted ones directly to your project using the tool's gradle plugin.
Please note that it will require you to create an account (for free) as it will sync any change to your project directly. (So you can add/change an image (svg, pdf, png, jpeg) and it will automatically add/change the converted files in your project at the next build.)
Currently it does not support 9 patch images.
(I should note that I worked on the tool and I think it is really cool and I use it personally. I have no shares in it.)
