codec2: do not count padding into allocation's capacity.

Padding is not usable so do not count it into an allocation's capacity.

At the same time document that fetchAllocation methods do not guarantee
exact capacity. Note, that in the future this behavior may be tightened
to guarantee this.

Bug: 187144478
Change-Id: I78f37971e959ea24f3ec689e790021984dfb74cb