I have a small update.
I found that Android has replaced its dx tool with another tool called d8.
I managed to plug it into my build script, and my findings are the
following:
- the .jar with Kawa version 1.13 still builds correctly
- when I try the latest Kawa built with javac --release 8 or --release 9,
the d8 complains:

Error: Field name '1+' cannot be represented in dex format.

I tried fixing that by renaming this function as add1 in the
gnu/commonlisp/lisp/primitives.lisp. Actually, there were several other
issues in that file (and it would probably be easier to be able to compile
Kawa without CommonLisp).

However, even after fixing that, I get another error:

Error: Field name '\=android\,view\,View' cannot be represented in dex
format.

I really don't know how to overcome that one (IIRC Kawa 1.13 used $Dt as a
replacement for a dot)