feat: volume fade in/out

[lovegaoshi]

supports the fade in/out feature request in RNTP. Because RN's Animated.Value won't change within Android's HeadlessJsTaskService, a native way to animate volume change to achieve a fading effect is necessary; I added a simple linear scaling that seems to work within the KA example.

scope.async doesn't work as I expected (Deferred.await() doesn't wait for RN's Promise.resolve?) any help is appreciated!