Fix the path for verity_key replacement when signing.

system_root_image expects the key at ROOT/verity_key as opposed to
BOOT/verity_key. Also refactor the verity key replacement lines.

Bug: 29397395
Test: 'sign_target_files_apks.py --replace_verity_private_key newkey --replace_verity_public_key newkey.pub target_files.zip signed-target_files.zip' and verify the replaced key in boot.img.
Change-Id: I58a5defff4be008ad55d4b5a5b7148569c3b8d66