Allow vendor_otatools arg to be file or folder

  Required by signer script when using "--vendor_otatools" argument.

Bug: 220126689
Bug: 186097910

Test: sign_target_files_apks \
        --vendor_otatools=otatools_vendor_dir \
	--vendor_partitions=vendor,odm \
	merged-target_files.zip \
	signed-target_files.zip

Change-Id: Iaa596f95f9db1e1e423b8306e9590fe27acb76e7