Create FieldPacker.getPos() to get the actual amount of data used for FP.

We need larger buffers to handle 64-bit RS object types, but 32-bit code
will never fill all of the getData().length bytes. This allows us to retain
our verification code.

We have to modify an existing use of FieldPacker that was doing partial
updates of the underlying buffer. This was really relying on the old data
to be implicitly selected and written back, but that is not guaranteed by
the original API. This also required a fix to the FieldPacker.reset() API,
which was not allowing the FieldPacker to ever point to the final entry in
its buffer.

Change-Id: Idcd52790ac2b0ab1eff3f043e7eec2832953f04b