vp10: code sign bit before absolute value in non-arithcoded header.

vp10: code sign bit before absolute value in non-arithcoded header.

For reading, this makes the operation branchless, although it still
requires two shifts. For writing, this makes the operation as fast
as writing an unsigned value, branchlessly. This is also how other
codecs typically code signed, non-arithcoded bitstream elements.

See issue 1039.

Change-Id: I6a8182cc88a16842fb431688c38f6b52d7f24ead