auto_updater: Modifying FixPayloads() method for LabTransfer usecase

This CL modifies the FixPayloads functionality for accommodating the
LabTransfer usecase.

BUG=chromium:1001312
TEST=Ran inside chroot: flash_unittest, uto_updater_transfer_unittest,
auto_updater_unnittest, manual testing by running provision_AutoUpdate
for versions 76, 77, and 80, cros flash <ip> <image>

Change-Id: I9efc4f3ec1c4e78d4d23ef585174b3e844a956ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1946598
Commit-Queue: Sanika Kulkarni <sanikak@chromium.org>
Tested-by: Sanika Kulkarni <sanikak@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Auto-Submit: Sanika Kulkarni <sanikak@chromium.org>
diff --git a/cli/flash_unittest.py b/cli/flash_unittest.py
index 7da089e..8a0077f 100644
--- a/cli/flash_unittest.py
+++ b/cli/flash_unittest.py
@@ -31,7 +31,8 @@
   """Mock out RemoteDeviceUpdater."""
   TARGET = 'chromite.lib.auto_updater.ChromiumOSUpdater'
   ATTRS = ('UpdateStateful', 'UpdateRootfs', 'SetupRootfsUpdate',
-           'RebootAndVerify', 'PreparePayloadPropsFile')
+           'RebootAndVerify', 'PreparePayloadPropsFile',
+           '_FixPayloadPropertiesFile')
 
   def __init__(self):
     partial_mock.PartialCmdMock.__init__(self)
@@ -51,6 +52,10 @@
   def PreparePayloadPropsFile(self, _inst, *_args, **_kwargs):
     """Mock out PreparePayloadPropsFile."""
 
+  def _FixPayloadPropertiesFile(self, _inst, *_args, **_kwargs):
+    """Mock out _FixPayloadPropertiesFile."""
+
+
 class RemoteAccessMock(remote_access_unittest.RemoteShMock):
   """Mock out RemoteAccess."""