Append root digest information of microdroid vendor into vendor.img

Instead appending public key information into vendor partition,
append root digest of microdroid-vendor's hashtree descriptor. The root
digest would be extracted in build time with avbtool.

Bug: 285855436
Test: avbtool info_image --image $ANDROID_PRODUCT_OUT/vendor/etc/avf/microdroid/microdroid_vendor.img
Test: avbtool info_image --image $ANDROID_PRODUCT_OUT/vendor.img
Change-Id: I930b09774a1024a2e5d30f474f1589fc1b8aa25a