Use java language level 8 for compiling module stubs

This is in line with the main stubs, which use 8. I believe using 9 is
unsuitable because it could generate bytecode that is incompatible with
some developer tooling stuck on 8. Either way, being consistent makes
sense.

Bug: 144149403
Test: diff_stubs.sh
Change-Id: Iddeceeebe9175e53e34f4b4194f01992e3083532