Add host side simulation python script

This enables host side simulation of partial updates with the original
complete target file

Bug: 171519321
Test: run this script with a partial update

     python3 simulate_ota.py --source
     ~/aosp/aosp_cf_x86_phone-target_files-6949164.zip --target
     ~/aosp/aosp_cf_x86_phone-target_files-6937553.zip
     ~/aosp/cf_x86_downgrade.zip

Change-Id: Iec52d8ac585b499d50bdb0c89d203ea353f2e94f
diff --git a/scripts/update_payload/payload.py b/scripts/update_payload/payload.py
index fe3a450..6b28046 100644
--- a/scripts/update_payload/payload.py
+++ b/scripts/update_payload/payload.py
@@ -124,8 +124,11 @@
     if zipfile.is_zipfile(payload_file):
       with zipfile.ZipFile(payload_file) as zfp:
         with zfp.open("payload.bin") as payload_fp:
-          payload_file = io.BytesIO(payload_fp.read())
-    self.payload_file = payload_file
+          self.payload_file = io.BytesIO(payload_fp.read())
+    elif isinstance(payload_file, str):
+      self.payload_file = open(payload_file, "rb")
+    else:
+      self.payload_file = payload_file
     self.payload_file_offset = payload_file_offset
     self.manifest_hasher = None
     self.is_init = False