CustomPip: fix next loosing video
When running in custom pip, video playback is not in the videoPlayerActivity. When starting next media, it checks if there is a video playing, and if (appIsStarted()) which returns false when customPip is running and the VideoPlayerActivity has been killed.
Because of this, when playing the following media in a playqueue with custom pip, there no vout. To fix this, this adds a check to service.isInPiPMode before removing video.
To reproduce the previous issue, just play a playqueue with more than one sample, leave the app with Custom PiP, and by the end of the sample, if the videoPlayerActivity has been killed, when changing media there should only be the last frame or a black screen.