Fix double-accounting bug in liblp.

When flashing in fastbootd, we create a new MetadataBuilder using the
given super_empty.img and attempt to import the existing partition
table. This will fail if there is some incompatibility in the partition
layout or partition quotas.

This import code was accidentally double-accounting partitions when
determining if they could fit within the group quota, preventing
"fastboot flashall" once partitions reached a certain size.

Bug: 126930319
Test: liblp_test gtest
Change-Id: I89a69cba110b62719197c9a4885cfc5bcf8f009f