Avoid concurrent modifications when notifying ViewTarget callbacks.

Fixes #2237.

library/src/main/java/com/bumptech/glide/request/target/ViewTarget.java

@@ -177,7 +177,11 @@ private static class SizeDeterminer {
     }
 
     private void notifyCbs(int width, int height) {
-      for (SizeReadyCallback cb : cbs) {
+      // One or more callbacks may trigger the removal of one or more additional callbacks, so we
+      // need a copy of the list to avoid a concurrent modification exception. One place this
+      // happens is when a full request completes from the in memory cache while its thumbnail is
+      // still being loaded asynchronously. See #2237.
+      for (SizeReadyCallback cb : new ArrayList<>(cbs)) {
         cb.onSizeReady(width, height);
       }
     }
@@ -218,6 +222,12 @@ void getSize(SizeReadyCallback cb) {
       }
     }
 
+    /**
+     * The callback may be called anyway if it is removed by another {@link SizeReadyCallback} or
+     * otherwise removed while we're notifying the list of callbacks.
+     *
+     * <p>See #2237.
+     */
     void removeCallback(SizeReadyCallback cb) {
       cbs.remove(cb);
     }