[Multimedia] Fixed minor issues and clean codes in AudioManager. (#82)

[Multimedia] Fixed minor issues and clean codes in AudioManager. (#82)

1. Adding null event handler - it should be prevented because if no handler is added before, then no need to register the native callback.
2. Renames error validation method to ThrowIfError for readability.
And some minor code improvements

Signed-off-by: coderhyme <jhyo.kim@samsung.com>