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

bp2build/conversion.go

@@ -20,7 +20,8 @@ type BazelFile struct {
 	Contents string
 }
 
-func CreateSoongInjectionFiles(cfg android.Config, metrics CodegenMetrics) []BazelFile {
+// PRIVATE: Use CreateSoongInjectionDirFiles instead
+func soongInjectionFiles(cfg android.Config, metrics CodegenMetrics) []BazelFile {
 	var files []BazelFile
 
 	files = append(files, newFile("android", GeneratedBuildFileName, "")) // Creates a //cc_toolchain package.