Create image apexes by default instead of flattened apexes

This is a list of all the internal master products that
use flattened apexes before and after this commit:
https://diff.googleplex.com/#key=AcyUcqOwqpXZ

Apexes can still be flattened by setting `TARGET_FLATTEN_APEX := true`
or `OVERRIDE_TARGET_FLATTEN_APEX := true` in their product config.

Bug: 254205429
Test: presubmits
Change-Id: I5b15f0fc9e149730fd33722632a8ac14432ab757