Change the condition for building super_empty.img.

This CL changes the condition for building super_empty.img from
PRODUCT_BUILD_SUPER_PARTITION to PRODUCT_USE_DYNAMIC_PARTITIONS, as a
follow-up to the change in [1].

With the CL in [1], it skips building super.img and super_empty.img both
when turning off PRODUCT_BUILD_SUPER_PARTITION. However, the latter
should be mandatory whenever dynamic partitions is enabled. Because
fastboot relies on this file to properly flash dynamic partitions. Plus,
the cost for building super_empty.img is much lower than the one for
super.img.

As part of the change, it'll write group info into target_files when
building with PRODUCT_BUILD_SUPER_PARTITION == false. It's the work for
target_files merging script to determine the values to be picked up. The
current logic in merge_target_files.py always uses the one from vendor
target_files. This CL adds a testcase to ensure the behavior.

[1] https://android-review.googlesource.com/c/platform/build/+/928756

Bug: 135752763
Test: `m dist` with a target that sets PRODUCT_BUILD_SUPER_PARTITION to
      false. Check the built artifacts contain super_empty.img. Verify
      that the build can be flashed properly.

Change-Id: I277f087eab45663a6c3b33333d16e9e576c1c25c
Merged-In: I277f087eab45663a6c3b33333d16e9e576c1c25c
(cherry picked from commit 48a2feb6)