writable-paths: add /usr/share/waydroid-extra

This can be used to easily setup unofficial images for testing e.g. newer Android versions etc.

https://docs.waydro.id/faq/using-custom-waydroid-images

Should this also be mirrored to ubports/xenial_-_android9 or is that some now unused branch (that should be removed)?

Closes #36 (closed).