releasetools: build_image.BuildVerityTree() returns a tuple.

We don't need verity_root_hash or verity_salt to be in the prop dict.

Test: `m dist` with aosp_marlin-userdebug
Test: python -m unittest test_verity_utils
Change-Id: I5a9c50f4741dfb1083b3f590136335b6bc0e5216
diff --git a/tools/releasetools/verity_utils.py b/tools/releasetools/verity_utils.py
index 0e605b1..38ebcf5 100644
--- a/tools/releasetools/verity_utils.py
+++ b/tools/releasetools/verity_utils.py
@@ -169,14 +169,17 @@
       self.image.WriteRangeDataToFd(self.hashtree_info.filesystem_range, fd)
 
     generated_verity_tree = common.MakeTempFile(prefix="verity")
-    prop_dict = {}
-    BuildVerityTree(adjusted_partition, generated_verity_tree, prop_dict)
+    root_hash, salt = BuildVerityTree(adjusted_partition, generated_verity_tree)
 
-    assert prop_dict["verity_salt"] == self.hashtree_info.salt
-    if prop_dict["verity_root_hash"] != self.hashtree_info.root_hash:
-      print("Calculated verty root hash {} doesn't match the one in metadata"
-            " {}".format(prop_dict["verity_root_hash"],
-                         self.hashtree_info.root_hash))
+    # The salt should be always identical, as we use fixed value.
+    assert salt == self.hashtree_info.salt, \
+        "Calculated salt {} doesn't match the one in metadata {}".format(
+            salt, self.hashtree_info.salt)
+
+    if root_hash != self.hashtree_info.root_hash:
+      print(
+          "Calculated root hash {} doesn't match the one in metadata {}".format(
+              root_hash, self.hashtree_info.root_hash))
       return False
 
     # Reads the generated hash tree and checks if it has the exact same bytes
@@ -190,6 +193,7 @@
 
     Returns:
       hashtree_info: The information needed to reconstruct the hashtree.
+
     Raises:
       HashtreeInfoGenerationError: If we fail to generate the exact bytes of
           the hashtree.