Remove sticky header on list reinitailization.

When we forward a message or share into the app, it is possible that we are going to reuse the same activity. In this case, when the adapter was reinitialized, we were just adding a new ItemDecoration every time.

This fix checks if we've already added one and removes it if necessary, just like the last seen decorator.

app/src/main/java/org/thoughtcrime/securesms/conversation/ConversationFragment.java

@@ -153,6 +153,7 @@ public class ConversationFragment extends LoggingFragment {
   private Locale                      locale;
   private RecyclerView                list;
   private RecyclerView.ItemDecoration lastSeenDecoration;
+  private RecyclerView.ItemDecoration stickyHeaderDecoration;
   private ViewSwitcher                topLoadMoreView;
   private ViewSwitcher                bottomLoadMoreView;
   private ConversationTypingView      typingView;
@@ -435,7 +436,7 @@ private void initializeListAdapter() {
       Log.d(TAG, "Initializing adapter for " + recipient.getId());
       ConversationAdapter adapter = new ConversationAdapter(GlideApp.with(this), locale, selectionClickListener, this.recipient.get());
       list.setAdapter(adapter);
-      list.addItemDecoration(new StickyHeaderDecoration(adapter, false, false));
+      setStickyHeaderDecoration(adapter);
       ConversationAdapter.initializePool(list.getRecycledViewPool());
 
       adapter.registerAdapterDataObserver(snapToTopDataObserver);
@@ -573,6 +574,15 @@ public void scrollToBottom() {
     }
   }
 
+  public void setStickyHeaderDecoration(@NonNull ConversationAdapter adapter) {
+    if (stickyHeaderDecoration != null) {
+      list.removeItemDecoration(stickyHeaderDecoration);
+    }
+
+    stickyHeaderDecoration = new StickyHeaderDecoration(adapter, false, false);
+    list.addItemDecoration(stickyHeaderDecoration);
+  }
+
   public void setLastSeen(long lastSeen) {
     if (lastSeenDecoration != null) {
       list.removeItemDecoration(lastSeenDecoration);