Add toggleMute method and handling corresponding key event

[elevenetc]

Added method which turns on and off sound.

[rzimmer]

Thanks a lot for your contribution! I still have to think about merging it as our goal is actually to be as close as possible to the official sources from Android and it's features (besides using TextureView, obviously). Hopefully I will come to a conclusion by the end of the year. Thanks for your patience!