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