Revert "Disable sparse images for A/B updater targets"

This reverts commit a927af67.

The original change breaks building vendor image for legacy A/B devices
with a recovery partition. Because build checks the out image size plus
the recovery patch size. If sparse images are disabled, then the output
image size will always be the max size. And max size plus anything will
always be greater than max size.

Change-Id: I87dd2bf78e9080d1fc2d320df7e98ee98efa031c