Convert to Android.bp

See build/soong/README.md for more information.

iw has been added to PRODUCT_PACKAGES_DEBUG in base_system.mk, since
debug tagging is not supported by Soong.

Test: cd external/iw; mma
Test: manually inspect version.c
Change-Id: I85af5606faff0209b22d6dc744315d3c05bbd234