gn2bp: add support for actions calling write_build_config.py

This is still incomplete, but should get us closer to what is needed.
A lot of the inputs to this genrule are generated by another genrule --
this will be a tough one.

Test: //base:jni_java__compile_java
Change-Id: Iae7e0d694a73deabed3f5f0592ed6ca0b17b09bd