releasetools: Support verity signer args.

system/extras/verity/build_verity_metadata.py now accepts
"--signer_args" to specify verity signer args.

Also remove the duplicate "--verity_signer_args" in
add_img_to_target_files.py, as we already have that in common.py.

Bug: 31500665
Test: Building and signing work w/ and w/o --signer_args.

Change-Id: I02f59c50a1ebf15c5505e9fffd5b9bbbbaa785be