liblp: Add MetadataBuilder::UsedSpace()

MetadataBuilder::AllocatableSpace() represents the total space
available.  Adding MetadataBuilder::UsedSpace() to represent the
space used by the existing set of partitions.  This will allow a
caller to predict if a partition can be resized (Grown), or to
create a partition to use up the free space (i.e. scratch).

Test: gTest liblp.UseAllDiskSpace
Bug: 109821005
Change-Id: Iae487d869fe18310c76b48c4281f1d6c1ee333cd