Updates:
Status: Invalid
Comment #2 on issue 285 by
jz...@google.com: Android *.9.png convert to webP
https://bugs.chromium.org/p/webp/issues/detail?id=285#c2Android Studio [1] skips creating .9's as webp as they seems to be unsupported [2]. Technically the image should decode, but I don't know enough about their use within Android to say whether it would be difficult to enable webp for them in a future android release. It may be worth a feature request on the Android Open Source project.
[1]
http://tools.android.com/tech-docs/webp[2] Android doesn't support 9-patch images from webp, so it will never convert .9.png images; the setting is there in the dialog to make it more obvious why these images are left unconverted.