Write the vbmeta digest to the output zipfile also

During signing, we write the entry directly to the output zip, intead
of a temp directory. Add the logic to write vbmeta_digest.txt to output
zipfile too. So the digest file will show up in the signed target files.

Bug: 189926233
Test: add_img_to_target_files -a <target-files.zip>
Change-Id: Ibf28a8f97512bda8c8c695e06190e1fb6573c53e