Don't sparse right-sized ext4 and erofs images.

When we introduced Dynamic Partitions, we stopped giving readonly
partitions fixed sizes. In addition we introduced deduplication for
ext4. These two factors greatly reduce the impact of sparse images,
since there aren't many fill blocks to optimize.

This patch disables sparsing for images that are rightsized and do not
explicitly specify extra reserved space. This makes the images a little
easier to work with from an engineering perspective. They no longer have
to be unsparsed to interact with any tooling. It also eases a potential
source of bugs, as b/184225422 is not reproducible with sparsing off.

On Pixel, the difference between the sparsed partitions and unsparsed is
12M (out of roughly 4G).

Bug: 198001223
Test: make, treehugger, make target-files-package
      dynamic partitions are no longer sparse images
Change-Id: I74459f8abe74a15a24ba5a40cf701e6af2db8179