sparse_img.py: Divide NONZERO blocks into groups.

For squashfs, we currently don't have a system.map. So the whole system
image will be treated as a single file. But for some unknown bug, the
updater will be killed due to OOM when writing back the patched image to
flash (observed on lenok-userdebug MEA49). Prior to getting a real fix,
we evenly divide the non-zero blocks into smaller groups (currently 1024
blocks or 4MB per group).

Bug: 23227672
Change-Id: Ifeddd8d802f01f8cd2a743a1d1217a284fb6e182