[Android] Fixes #2480, music does not pause when process backgrounded

This modifies SDL_PauseAudio behavior to pause all audio devices instead of
just the default one (required on Android, at least for testmultiaudio on my
Nexus 4 which reported 2 audio devices).
It also changes SDL_PauseAudioDevice to retain the device lock from pause until
resume in order to save battery in mobile devices.