liblp: Add a gtest for sparse image creation.

Note that this moves SparseBuilder into images.h, and splits
ReadLogicalPartitionGeometry into componenet methods for better
testability.

Bug: 116802789
Test: liblp_test gtest
Change-Id: Ib41a46b633c71623c136a10fcc8017e4de20884c
6 files changed