Update MCSystemSound{Player,Channel} to use StringRef names

The class MCSoundFinishedOnChannelEvent and the function MCSoundPostSoundFinishedOnChannelMessage have also been updated.

Both Android and iOS have been changed.
Minor fixes

Tidy up a JNI function by using MCJavaStringToStringRef and remove an un-neccessary check for empty names.
Merge pull request #175 from runrevfraser/refactor-syntax_unicode_mblsound

Refactor syntax unicode mblsound