Override descriptors of the same partition.

When using multiple --include_descriptors_from_image, if there are
multiple descriptors for the same partition, only include the last
one, this is useful if we want to update one descriptor in an existing
vbmeta.img.

Bug: 76386656
Test: avbtool info_image --image $OUT/vbmeta_dev.img
Test: libavb_host_unittest
Change-Id: I7cfeef79013355ca4858ccf99798b76049674bd7