Support selecting audio tracks

Implements audio track selection on iOS & Android ExoPlayer. The prop mirrors the API for selectedTextTrack.

Video.js

@@ -316,6 +316,13 @@ Video.propTypes = {
   posterResizeMode: Image.propTypes.resizeMode,
   repeat: PropTypes.bool,
   allowsExternalPlayback: PropTypes.bool,
+  selectedAudioTrack: PropTypes.shape({
+    type: PropTypes.string.isRequired,
+    value: PropTypes.oneOfType([
+      PropTypes.string,
+      PropTypes.number
+    ])
+  }),
   selectedTextTrack: PropTypes.shape({
     type: PropTypes.string.isRequired,
     value: PropTypes.oneOfType([