fastboot: Fix flashing failure when a sparse image has an avb footer.

Trying to move the AVB footer on a sparse file will corrupt the sparse
format. Rather than implement this properly, for now, have the
copy_avb_footer() function gracefully fail by skipping the operation.

Bug: 304574023
Test: fastboot flash sparse image with avb footer
Change-Id: Ia6f0711789a04897ec266ad604a3d243c7184082