Fix flash of audio-only poster in cast receiver
Fix flash of audio-only poster in cast receiver

The audio-only poster was showing before there was really anything
loaded.

Change-Id: Icc62e3dd90b748491444150c16ccbb3d69081a7c

shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player