Build vendor images first when rebuilding with vendor_otatools.

Only supports rebuilding vendor.img and odm.img currently.

This allows "other" images (e.g. vbmeta, super_empty, ...) to
be rebuilt using the latest tooling.

Bug: 188491126
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 \
	merged-target_files.zip \
	signed-target_files.zip
Change-Id: Ib93aea9f083ee9b722c31a42dcc780b5222053b8