Unify the get_layers calls across all implements of manifest schemas to ensure we have a common type returned

Also renames some methods to make it more clear what kind of information they return

workers/manifestbackfillworker.py

@@ -47,9 +47,8 @@ class BrokenManifest(ManifestInterface):
   def bytes(self):
     return self._payload
 
-  @property
-  def layers(self):
-    return []
+  def get_layers(self, content_retriever):
+    return None
 
   def get_legacy_image_ids(self, cr):
     return []