[iOS] Adjust setRate() behavior to match Android.
[iOS] Adjust setRate() behavior to match Android.

Setting the rate will no longer cause the track to start playing.

After using setRate(), tracks will play at this rate until the rate is changed again.

Fix #766
Fix #614
Fix #520

react-native-kit/react-native-track-player,react-native-kit/react-native-track-player,react-native-kit/react-native-track-player,react-native-kit/react-native-track-player,react-native-kit/react-native-track-player