liblp: Move backup sectors to the start of the partition.

Previously, metadata backups were stored at the end of the partition to
make them easy to locate. On older devices where the super partition
could span system/vendor partitions, we may want to leave the end of
each partition free to store an AVB footer. To allow this, we now store
geometry and metadata backups near the start of the partition instead.
They are still positioned at a fixed offset.

Bug: 116802789
Test: device boots after flashing new metadata
Change-Id: Ib173f251a4a13e290adcc4ac5cfbeb030eacda30