Rename ota_utils.Payload to PayloadGenerator

There's also a Payload class in system/update_engine/scripts. To avoid
name collision, rename the one in releasetools to PayloadGenerator.
No functional changes in this CL.

Bug: 227848550
Test: th

Change-Id: Ib7d4c7ad9839d99416d965e3a3661b8cee7b7693
diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index 7aada51..ab65ee2 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -263,7 +263,7 @@
 import common
 import ota_utils
 from ota_utils import (UNZIP_PATTERN, FinalizeMetadata, GetPackageMetadata,
-                       Payload, SECURITY_PATCH_LEVEL_PROP_NAME, StreamingPropertyFiles, AbOtaPropertyFiles)
+                       PayloadGenerator, SECURITY_PATCH_LEVEL_PROP_NAME, StreamingPropertyFiles, AbOtaPropertyFiles)
 from common import IsSparseImage
 import target_files_diff
 from check_target_files_vintf import CheckVintfIfTrebleEnabled
@@ -893,7 +893,7 @@
   # Metadata to comply with Android OTA package format.
   metadata = GetPackageMetadata(target_info, source_info)
   # Generate payload.
-  payload = Payload()
+  payload = PayloadGenerator()
 
   partition_timestamps_flags = []
   # Enforce a max timestamp this payload can be applied on top of.
@@ -972,7 +972,7 @@
     # building an incremental OTA. See the comments for "--include_secondary".
     secondary_target_file = GetTargetFilesZipForSecondaryImages(
         target_file, OPTIONS.skip_postinstall)
-    secondary_payload = Payload(secondary=True)
+    secondary_payload = PayloadGenerator(secondary=True)
     secondary_payload.Generate(secondary_target_file,
                                additional_args=["--max_timestamp",
                                                 max_timestamp])