[boblight-aml] - fixed - poweroff the leds via priority 255 instead of s...

...ending a black frame - also set priority to 255 if no frames are captured for letting other clients handle the leds.

This allows the following scenario:
1. start boblight-aml with -p 100 which makes it priority 100
2. also enable the boblight addon in xbmc/kodi with static backlight.

Now whenever you start a movie - boblight-aml will rule the leds. Once playback is stopped the boblight addon gains power over the leds again and can do the static light in the menus. This is possible because boblight addon uses default priority 128

Prio is
100 - boblight-aml
128 - boblight addon
255 - leds off