Make makedict able to write binary format versions 1 and 2

Version 2 provides for a variable-length header region.
The first step is to allow the implementation to simply step
over the header, which will allow us to put any ignored metadata
we want. This change implements the makedict part of this.

Bug: 5686638
Change-Id: I97f1a8a4f84d15e1bedbad74e54303852aef6e0b