(android)Modify Play Audio Mechanism

On android, play audio is by creating a new thread(SDL_RunAudio), once app
go into background, the thread will be suspended, result in app can not play
audio when background.

To support play audio when background, I modified the audio play mechanism,
it uses Service(). By the way, add a hint(SDL_HINT_BACKGROUND_AUDIO) whether
to allow the play audio when the background.

Source code on Github: https://github.com/freeors/SDL