Fix potential NPE/ThreadSafety issues with MediaDrm listener

- MediaDrmEventListener.onEvent is typically called on the
  app's main thread. mediaDrmHandler is instantiated on the
  playback thread. Hence mediaDrmHandler should be volatile
  to ensure visibility.
- Nulling mediaDrmHandler could result in a NPE in onEvent.
  Instantiate mediaDrmHandler (and don't null it again) to
  avoid this. MediaDrmHandler.handleMessage will correctly
  discard any events for sessions that are now closed.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168707938

library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSessionManager.java

@@ -98,21 +98,22 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
   /** Releases an existing offline license. */
   public static final int MODE_RELEASE = 3;
 
-  private final Handler eventHandler;
-  private final EventListener eventListener;
+  private final UUID uuid;
   private final ExoMediaDrm<T> mediaDrm;
-  private final HashMap<String, String> optionalKeyRequestParameters;
   private final MediaDrmCallback callback;
-  private final UUID uuid;
+  private final HashMap<String, String> optionalKeyRequestParameters;
+  private final Handler eventHandler;
+  private final EventListener eventListener;
   private final boolean multiSession;
 
+  private final List<DefaultDrmSession<T>> sessions;
+  private final AtomicBoolean provisioningInProgress;
+
   private Looper playbackLooper;
   private int mode;
   private byte[] offlineLicenseKeySetId;
 
-  private final List<DefaultDrmSession<T>> sessions;
-  private final AtomicBoolean provisioningInProgress;
-  /* package */ MediaDrmHandler mediaDrmHandler;
+  /* package */ volatile MediaDrmHandler mediaDrmHandler;
 
   /**
    * Instantiates a new instance using the Widevine scheme.
@@ -226,6 +227,7 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
     if (multiSession) {
       mediaDrm.setPropertyString("sessionSharing", "enable");
     }
+    mediaDrm.setOnEventListener(new MediaDrmEventListener());
   }
 
   /**
@@ -334,8 +336,9 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
     Assertions.checkState(this.playbackLooper == null || this.playbackLooper == playbackLooper);
     if (sessions.isEmpty()) {
       this.playbackLooper = playbackLooper;
+      if (mediaDrmHandler == null) {
         mediaDrmHandler = new MediaDrmHandler(playbackLooper);
-      mediaDrm.setOnEventListener(new MediaDrmEventListener());
+      }
     }
 
     DefaultDrmSession<T> session = null;
@@ -383,13 +386,6 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
     if (drmSession.release()) {
       sessions.remove(drmSession);
     }
-
-    if (sessions.isEmpty()) {
-      mediaDrm.setOnEventListener(null);
-      mediaDrmHandler.removeCallbacksAndMessages(null);
-      mediaDrmHandler = null;
-      playbackLooper = null;
-    }
   }
 
   @Override