Update DrmSessionException.

Make DrmSessionException takes in Throwable cause instead of Exception cause, which is more limiting and doesn't add any benefit.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160121486

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

@@ -28,11 +28,11 @@ import java.util.Map;
 @TargetApi(16)
 public interface DrmSession<T extends ExoMediaCrypto> {
 
-  /** Wraps the exception which is the cause of the error state. */
+  /** Wraps the throwable which is the cause of the error state. */
   class DrmSessionException extends Exception {
 
-    public DrmSessionException(Exception e) {
-      super(e);
+    public DrmSessionException(Throwable cause) {
+      super(cause);
     }
 
   }