Allow to build non-sparse super image.

- Rename SparseBuilder to ImageBuilder, WriteSplitSparseFiles
  to WriteSplitImageFiles, WriteToSparseFile to WriteToImageFile, etc.
- Add sparsify argument to ImageBuilder, WriteToImageFile, and
  WriteSplitImageFiles

This allows lpmake to write non-sparse super image directly. Virtual
devices needs non-sparse images to work on.

Test: lpmake without --sparse
Test: liblp_test
Bug: 120041578

Change-Id: I76ee09efd02df7caaa8abd37a55ae0bebf9cfa29