libvpx

Change the use of a reserved color space entry

This commit rename a reserved color space entry to BT_2020, it intends
to provide support for VP9 bitstream to pass along the color space
type defined in BT.2020(Rec.2020)

please note this entry does not have any effect on encoding/decoding
behavior, but allow applications to the pass the information along
from encoding end to decoding end.

Change-Id: I4678520e89141ea5e8900f7bd1c0e95b710b7091