bp2build converts java_genrule and java_genrule_host

Bp2build currently supports building genrules for cc modules, but does
not support building java_genrule* modules. This commit adds this
functionality.

Bug: 213480907
Test: go test ./bp2build
Change-Id: I473196c5bcf5582ba0c8faa65b5005f81ac973a4