liblp: Allocate "b" extents in the second half of super.

When allocating "b" partitions on a non-retrofit A/B device, prioritize
regions occuring in the second half of the super partition. To make this
effective, the region covering the midpoint sector is split into two
additional regions.

This will allow OTAs to avoid unecessary fragmentation, since each
slot's partitions will be grouped together, leaving a large chunk of
contiguous space available when the OTA deletes the target slot. Since
updates are not allowed to consume more than half of the super
partition, this should guarantee one extent per partition. Note that, if
this restriction is not in place (for example, a developer flashes a
massive "system_b"), then an additional extent will be allocated due to
the region that was split.

Bug: 120433288
Test: liblp_test gtest
Change-Id: I1797e59e14c8b0d4d0e6855a1d984e8159b21df2