rusty-gd: handle padding in sizes

need to unroll size concatenation, to ensure padding only applies to
size contributions *before* it exists

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost DirectHciTest
Change-Id: If7eabc0b1f499c2168fec6df6c5e294c5672b8e4