liblp: Simplify GrowPartition().

The partition resize algorithm duplicates a lot of logic because it
handles the final free interval separately from other free intervals.
This is unnecessary and makes it harder to change the actual algorithm.

This change makes GrowPartition() treat the final free space region the
same as free gaps in between partitions. It does this by converting the
extent list into a gap list, and then adds a final gap for the remainder
of the free space. The resize function no longer has to treat the end of
the disk separately.

This patch does not change the way partitions are allocated, it is
purely a refactoring.

Bug: 79173901
Test: liblp_test gtest
Change-Id: I4780f20b23fe021eac62de874b061857712c04fe