protected synchronized @Nullable Multiplexer getExistingMultiplexer(K key) {
    return mMultiplexers.get(key);
  }
Map of multiplexers guarded by "this" lock. The lock should be used only to synchronize
accesses to this map. In particular, no callbacks or third party code should be run under
"this" lock.

<p>The map might contain entries in progress, entries in progress for which cancellation has
been requested and ignored, or cancelled entries for which onCancellation has not been called
yet.
imagepipeline/src/main/java/com/facebook/imagepipeline/producers/MultiplexProducer.java