Disable copying images for target_files.zip

For the OTA build, target_files.zip requires setting fixed timestamp
for images. However, it caused regression on 'adb sync'.
Restoring image build process for target_files.zip instead of copying
them from the PRODUCT_OUT, only the images in target_files.zip set the
fixed timestamp.
By setting COPY_IMAGES_FOR_TARGET_FILES_ZIP flag, we still may copy
the images for target_files.zip for future use.

Bug: 287534409
Test: m; flashall; adb sync
Test: m dist; flash images from target_files.zip and check the timestamp
Change-Id: I4135eb721c33581f191621847e6595833263d707

tools/releasetools/build_image.py

@@ -683,7 +683,9 @@ def ImagePropFromGlobalDict(glob_dict, mount_point):
   d = {}
   TryParseFingerprint(glob_dict)
 
-  d["timestamp"] = FIXED_FILE_TIMESTAMP
+  # Set fixed timestamp for building the OTA package.
+  if "use_fixed_timestamp" in glob_dict:
+    d["timestamp"] = FIXED_FILE_TIMESTAMP
   if "build.prop" in glob_dict:
     timestamp = glob_dict["build.prop"].GetProp("ro.build.date.utc")
     if timestamp: