gn2bp: stop parsing dep tree for builtin modules

Targets that are part of builtin_deps will be removed / replaced by
gen_android_bp. So if such a target is encountered while parsing the gn
desc output, all of its dependencies can be safely ignored.

Test: builds
Change-Id: I5bd954c2b95e2ae53a4ac9dc420a067fd76da9a7
3 files changed