Support middleman actions in mixed builds

This allows support of generated hdrs / srcs in mixed builds.

Test: Manually verified that libc_bionic_ndk passes compilation (failed
previously due to missing generated heaer)
Test: bp2build and mixed_libc CI scripts
Test: New aquery test

Change-Id: I88e359a4bd9eba383c207d5cf812272725ff0a3d