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