MediaRouter: Tweak the volume item height to conform to the spec

In the spec, in 4 items case, the volume list should be 288dp and
fully cover the artwork. This change tweaks the height to 68dp.
(68dp * 4 + 16dp = 288dp)

Change-Id: I5126bcb48601667db870e78835d3f9e3f537f895
1 file changed