Revert ^2 "Prevent unspecified values in soong_config_string_variable""

This reverts commit 38944c70.

Reason for revert: I believe we're ready for this now, but run build_test 1-4 on internal master for at least master and tm-dev-plus-aosp on go/abtd before submitting

Change-Id: Id3bcd9a46f3087cf2d34dece5294828ea9436788