Fix the following issues mentioned in Pixel SBOM review.

1) PackageSupplier should be NOASSERTION if there is no homepage
   information in METADATA file of source packages
2) PackageDownloadLocation of upstream packages should be NOASSERTION if
   there is no code repository URL in METADATA file of source packages

Test: CIs
Test: atest --host sbom_writers_test
Change-Id: I8a0298b7bacc2f96555f9d7dde0d21ada8c6b564

tools/sbom/sbom_writers.py

@@ -86,7 +86,7 @@ class TagValueWriter:
 
   @staticmethod
   def marshal_package(package):
-    download_location = 'NONE'
+    download_location = sbom_data.VALUE_NOASSERTION
     if package.download_location:
       download_location = package.download_location
     tagvalues = [
@@ -296,7 +296,7 @@ class JSONWriter:
       package = {
         PropNames.NAME: p.name,
         PropNames.SPDXID: p.id,
-        PropNames.PACKAGE_DOWNLOAD_LOCATION: p.download_location if p.download_location else 'NONE',
+        PropNames.PACKAGE_DOWNLOAD_LOCATION: p.download_location if p.download_location else sbom_data.VALUE_NOASSERTION,
         PropNames.FILES_ANALYZED: p.files_analyzed
       }
       if p.version: