Add a new CreateSoongInjectionDirFiles function

- This is a wrapper function for all files in soong_injection directory.
This should prevent an error in the workspace generated for
api_bp2build.
- Rename the existing CreateSoongInjectionFiles fn and make it package
  private to prevent confusion

(The subsequent CL in this stack should contain a smoke test for
api_bp2build)

Test: b build //:empty --config=api_bp2build --config=android
Test: TH
Change-Id: Iddb0aa1aff2f709826edd587aa99fccddf80f08f