VEX-7093: Protect initialization of DRM (#29)

This PR will fix the crash from the experiment on react-native-video

Jira: VEX-7093

velocity PR: crunchyroll/velocity#2443

The crash was caused by using the player object before it was set by another thread, the change simply waits for the player to be available before executing any player interaction after DRM initialization.

Reviews
Major reviewer (domain expert): @jctorresM
Minor reviewer: @jacob-livingston

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

@@ -632,6 +632,16 @@ private void initializePlayerSource(ReactExoplayerView self, DrmSessionManager d
             mediaSource = new MergingMediaSource(textSourceArray);
         }
 
+        // wait for player to be set
+        while (player == null) {
+            try {
+                wait();
+            } catch (InterruptedException ex) {
+                Thread.currentThread().interrupt();
+                Log.e("ExoPlayer Exception", ex.toString());
+            }
+        }
+
         boolean haveResumePosition = resumeWindow != C.INDEX_UNSET;
         if (haveResumePosition) {
             player.seekTo(resumeWindow, resumePosition);