Fix issue where scrolling could get stuck.

The number of off-screen pages was too small, resulting in the
possibility of you still being offscreen after the pages loaded,
which could lead to loading more data, which could lead to you still
being offscreen, ad infinitum.

Simply increasing the number of buffer
pages resolves it.

Tested by adding an artificial 1 second delay to
loading a page.

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

@@ -93,7 +93,7 @@ class ConversationViewModel extends ViewModel {
       ConversationDataSource dataSource = new ConversationDataSource(context, data.getThreadId());
       PagingConfig           config     = new PagingConfig.Builder()
                                                           .setPageSize(25)
-                                                          .setBufferPages(1)
+                                                          .setBufferPages(3)
                                                           .setStartIndex(Math.max(startPosition, 0))
                                                           .build();