VEX-5044: Allow exoplayer to preinit with empty source (#6)

- Allow player to be init before source is provided, and later update once a source is provided.
- Adds handling for providing a empty source in order to stop playback and clear out any existing content

android-exoplayer/src/main/java/com/brentvatne/exoplayer/ReactExoplayerViewManager.java

@@ -147,6 +147,7 @@ public class ReactExoplayerViewManager extends ViewGroupManager<ReactExoplayerVi
         Map<String, String> headers = src.hasKey(PROP_SRC_HEADERS) ? toStringMap(src.getMap(PROP_SRC_HEADERS)) : null;
 
         if (TextUtils.isEmpty(uriString)) {
+            videoView.clearSrc();
             return;
         }