Append pubkey of microdroid-vendor into avb prop of vendor_boot

For protected VM in AVF, the public key of microdroid-vendor partition
should be passed via trustable way. However, pvmfw config data is
constructed from ABL. AVB property is needed to pass the public key from
the source code into ABL, determined in the build time.

Bug: 285854379
Test: avbtool info_image --image $ANDROID_PRODUCT_OUT/vendor_boot.img
Change-Id: I024d1d835cccb27feef1164aca30712dbb8a70da