SystemUI: Make lockscreen visualizer battery friendly

In case of offloaded playback, the device is allowed to sleep.
However, if the lockscreen visualizer is enabled, the device
hardly enters deep sleep, even if the screen is off. To prevent
this, enable the visualizer only when the lockscreen is shown.

Change-Id: I484270694b734c9b53e61a7c0ad74391b54fe8cd