Rebuild vendor images in sign_target_files_apks with vendor otatools.

This is needed until image compilation tooling is backwards compatible.

Bug: 188491126
Bug: 192253131
Bug: 192422274
Test: Sign a GRF R+S merged target_files package using:
      sign_target_files_apks \
        --vendor_otatools=<otatools.zip from R> \
	--vendor_partitions=vendor,odm,vbmeta \
	merged-target_files.zip \
	signed-target_files.zip
Change-Id: Iec208f544b56f8ed577344d8d6ca904773d8baab