liblp: Store device information in a new block device table.

This patch removes the alignment, block device size, and starting sector
fields from LpGeometry into a new LpMetadataBlockDevice struct. The
metadata now contains a table of these structs, and the table will have
exactly one entry representing the super partition.

This refactoring will make it easier to have logical partitions span
multiple physical partitions. When that happens, the table will be
allowed to have more than one entry, and the first entry of the table
will be considered the "root" of the super partition.

Bug: 116802789
Test: liblp_test gtest
      device with logical partitions flashes and boots
Change-Id: I97f23beac0363182cb6ae78ba2595860950afcf0