Avoid feeding D8 with dex files.

Some test targets include dex files as resource to be loaded
at runtime. This is an issue since full D8 also take dex files
as input.

This CL extracts only the class files and feed that to D8 instead
of the whole jar.

Test: USE_D8_DESUGAR=true m checkbuild tests
Bug: 69329508

Change-Id: I52d80ec01933573f18eb497472a80b6a649c9dae