Explicitly null out LoadTask.callback on release

As highlighted by the ref'd issue, we can end up with
memory leaks if Loadable.load implementations take a long
time to return upon cancelation. This change cuts off one
of the two problematic reference chains.

This doesn't do much about the ref'd issue, since there's
a second reference chain that's much harder to deal with:
Thread->LoadTask->loadable. But since it's easy just to
cut this one off, I figure it makes sense to do so.

Issue: #4249

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198735386

library/core/src/main/java/com/google/android/exoplayer2/upstream/Loader.java

@@ -250,11 +250,12 @@ public final class Loader implements LoaderErrorThrower {
     private static final int MSG_IO_EXCEPTION = 3;
     private static final int MSG_FATAL_ERROR = 4;
 
-    private final T loadable;
-    private final Loader.Callback<T> callback;
     public final int defaultMinRetryCount;
+
+    private final T loadable;
     private final long startTimeMs;
 
+    private @Nullable Loader.Callback<T> callback;
     private IOException currentError;
     private int errorCount;
 
@@ -304,6 +305,11 @@ public final class Loader implements LoaderErrorThrower {
         finish();
         long nowMs = SystemClock.elapsedRealtime();
         callback.onLoadCanceled(loadable, nowMs, nowMs - startTimeMs, true);
+        // If loading, this task will be referenced from a GC root (the loading thread) until
+        // cancellation completes. The time taken for cancellation to complete depends on the
+        // implementation of the Loadable that the task is loading. We null the callback reference
+        // here so that it doesn't prevent garbage collection whilst cancellation is ongoing.
+        callback = null;
       }
     }