Auto initialize update payload object

It's not pythonitc to ask users to call payload.Init() after creating a
payload object. Move all the initialization work to __init__. To keep
existing code compatible, Init() function is implemented as no op.

Test: th
Bug: 227848550
Change-Id: I1a01051acc274111a096470a2b9d7a6baaa6c24e
diff --git a/scripts/update_payload/payload.py b/scripts/update_payload/payload.py
index 5fd47c1..e8f0f7e 100644
--- a/scripts/update_payload/payload.py
+++ b/scripts/update_payload/payload.py
@@ -141,6 +141,7 @@
     self.metadata_signature = None
     self.payload_signature = None
     self.metadata_size = None
+    self.Init()
 
   @property
   def is_incremental(self):
@@ -223,7 +224,7 @@
       correctly.
     """
     if self.is_init:
-      raise PayloadError('payload object already initialized')
+      return
 
     self.manifest_hasher = hashlib.sha256()