androidenv: fix system images xsl used in generate.sh

Updated with fixes for `convertsystemimages.xsl`:
- Use `type-details/codename` if it exists, falling back to
  `type-details/api-level`: this results in "Q" rather than "28" for
  preview images
- Use `<xsl:text>` elements to control whitespace in the output.

This is extracted from https://github.com/NixOS/nixpkgs PR 58131.