SDK / exoplayer

The SDR constant also specified a color space and range, in addition to
C.COLOR_TRANSFER_SDR. However, it turns out that SDR videos may use different color
space and range values, so following prior ExoPlayer conventions to have `null`
mean "generic SDR" is preferable here.

PiperOrigin-RevId: 459296746
(cherry picked from commit 4230ea94)