Put DownloadTasks on hold until preceding conflicting tasks are complete

Tasks conflict if both of them work on the same media and at least one
of them is remove action.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172741795

library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadException.java

@@ -25,4 +25,9 @@ public final class DownloadException extends IOException {
     super(message);
   }
 
+  /** @param cause The cause for the exception. */
+  public DownloadException(Throwable cause) {
+    super(cause);
+  }
+
 }

library/core/src/main/java/com/google/android/exoplayer2/util/ConditionVariable.java

@@ -16,8 +16,8 @@
 package com.google.android.exoplayer2.util;
 
 /**
- * A condition variable whose {@link #open()} and {@link #close()} methods return whether they
- * resulted in a change of state.
+ * An interruptible condition variable whose {@link #open()} and {@link #close()} methods return
+ * whether they resulted in a change of state.
  */
 public final class ConditionVariable {
 
@@ -59,4 +59,21 @@ public final class ConditionVariable {
     }
   }
 
+  /**
+   * Blocks until the condition is opened or until timeout milliseconds have passed.
+   *
+   * @param timeout The maximum time to wait in milliseconds.
+   * @return true If the condition was opened, false if the call returns because of the timeout.
+   * @throws InterruptedException If the thread is interrupted.
+   */
+  public synchronized boolean block(int timeout) throws InterruptedException {
+    long now = System.currentTimeMillis();
+    long end = now + timeout;
+    while (!isOpen && now < end) {
+      wait(end - now);
+      now = System.currentTimeMillis();
+    }
+    return isOpen;
+  }
+
 }