Fix a failure in signing test

Pixel moved away from sparse images, so validate_target_files is failing
because it expects sparse images.

Test: th
Change-Id: I322ff10c2afbacfb4d78991be60c11aac92a6d4c

tools/releasetools/ota_from_target_files.py

@@ -251,6 +251,7 @@ import common
 import ota_utils
 from ota_utils import (UNZIP_PATTERN, FinalizeMetadata, GetPackageMetadata,
                        PropertyFiles, SECURITY_PATCH_LEVEL_PROP_NAME, GetZipEntryOffset)
+from common import IsSparseImage
 import target_files_diff
 from check_target_files_vintf import CheckVintfIfTrebleEnabled
 from non_ab_ota import GenerateNonAbOtaPackage
@@ -1021,13 +1022,6 @@ def GeneratePartitionTimestampFlagsDowngrade(
   ]
 
 
-def IsSparseImage(filepath):
-  with open(filepath, 'rb') as fp:
-    # Magic for android sparse image format
-    # https://source.android.com/devices/bootloader/images
-    return fp.read(4) == b'\x3A\xFF\x26\xED'
-
-
 def SupportsMainlineGkiUpdates(target_file):
   """Return True if the build supports MainlineGKIUpdates.