updater_sample: tools: import AbOtaPropertyFiles from ota_utils

Commit [1] moved the AbOtaPropertyFiles class from the ota_from_target_files
file to the ota_utils file.

Because of this move, running gen_update_config.py breaks:
AttributeError: module 'ota_from_target_files' has no attribute 'AbOtaPropertyFiles'

Fix the error by importing from ota_utils instead of from
ota_from_target_files.

[1] https://android-review.googlesource.com/c/platform/build/+/2201755
Change-Id: Ife3d26bdfdeedf26888cd311ed490edf73c103da
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/updater_sample/tools/gen_update_config.py b/updater_sample/tools/gen_update_config.py
index f2cb1a8..5ff3b89 100755
--- a/updater_sample/tools/gen_update_config.py
+++ b/updater_sample/tools/gen_update_config.py
@@ -32,7 +32,7 @@
 import sys
 import zipfile
 
-import ota_from_target_files  # pylint: disable=import-error
+import ota_utils  # pylint: disable=import-error
 
 
 class GenUpdateConfig(object):
@@ -100,7 +100,7 @@
     def _get_property_files(package_zip):
         """Constructs the property-files list for A/B streaming metadata."""
 
-        ab_ota = ota_from_target_files.AbOtaPropertyFiles()
+        ab_ota = ota_utils.AbOtaPropertyFiles()
         property_str = ab_ota.GetPropertyFilesString(package_zip, False)
         property_files = []
         for file in property_str.split(','):