liblp: Allow specifying a custom block size when building sparse images.

liblp currently creates sparse files with a block size of 512 bytes.
Unfortunately devices can reject sparse files that don't have a specific
block size. To accomodate this, SparseBuilder now requires that
partition metadata be built with an explicit block size, and the device
alignment must be a multiple of that block size.

This change also modifies SerializeGeometry to automatically include
0-padding to 4096 bytes.

Bug: 79173901
Test: lpmake -S can be flashed with fastboot to walleye
Change-Id: Iab04f6f55bb373e21767963335c85ad1119a65a7