libbinder: Remove flexible array from RpcWireReply

We are going to change the size of this struct depending on the protocol
version and that gets messy when there is a flexible array member.

We could remove it from RpcWireTransaction as well, but that is a bigger
change and there is no motivation yet (besides consistency).

This change also happens to optimize out one allocation when the reply
parcel is zero bytes.

Bug: 185909244
Test: TH
Change-Id: I18a5712ba80e7b311b945ef54977b66ffa43e1ca