Add function to compare the partitions' extents in metadata

For partial updates, the metadata for untouched dynamic partitions
are just copied over to the target slot. So, verifying the extents
of these partitions in the target metadata should be sufficient for
correctness. And we don't need to read & hash the bytes on these
partitions.

Bug: 151088567
Test: unit tests pass
Change-Id: I95836ee6f76d884c7a1f5537154ac7230563493a