liblp: Improve reliability of UpdatePartitionTable.

If UpdatePartitionTable is interrupted while writing metadata, the next
update becomes much more risky, as only one valid copy may exist. If
that subsequent update is interrupted, all metadata copies may be
corrupt.

To alleviate this, we now synchronize both metadata copies before each
update. If the backup copy is corrupted, we replace it with the primary
copy, and vice versa. Similarly if the primary and backup metadata do
not match, we sync the backup to contain the same data as the primary.

If for some reason this synchronization process fails, we do not proceed
to write a new partition table.

Bug: 79173901
Test: liblp_test gtest
Change-Id: Ic80cf9a5dc6336ff532e069ca5c8c76371550cb9