fix images disappearing when in recyclerviews

Fixes #3142
Closes #3288
// FREEBIE

src/org/thoughtcrime/securesms/ConversationItem.java

@@ -166,6 +166,7 @@ public class ConversationItem extends LinearLayout {
   }
 
   public void unbind() {
+    mediaThumbnail.clear();
   }
 
   public MessageRecord getMessageRecord() {

src/org/thoughtcrime/securesms/components/ThumbnailView.java

@@ -53,11 +53,6 @@ public class ThumbnailView extends ForegroundImageView {
     super(context, attrs, defStyle);
   }
 
-  @Override protected void onDetachedFromWindow() {
-    Glide.clear(this);
-    super.onDetachedFromWindow();
-  }
-
   public void setImageResource(@Nullable MasterSecret masterSecret,
                                long id, long timestamp,
                                @NonNull ListenableFutureTask<SlideDeck> slideDeckFuture)
@@ -97,6 +92,10 @@ public class ThumbnailView extends ForegroundImageView {
     this.thumbnailClickListener = listener;
   }
 
+  public void clear() {
+    if (isContextValid()) Glide.clear(this);
+  }
+
   @TargetApi(VERSION_CODES.JELLY_BEAN_MR1)
   private boolean isContextValid() {
     return !(getContext() instanceof Activity)            ||