title: Restore PagedListAdapter position when resuming activity
body: <p>I've been experimenting with <code>PagedListAdapter</code> and can't figure out how to restore adapters position correctly.</p>
  
  <p>Last attempt was to save <code>lastKey</code> from current list.</p>
  
  <pre><code>override fun onSaveInstanceState(outState: Bundle) {
      super.onSaveInstanceState(outState)
  
      val lastKey = adapter.currentList?.lastKey as Int
  
      outState.putInt("lastKey", lastKey)
  }
  </code></pre>
  
  <p>but when restoring my adapter and passing <code>lastKey</code> to <code>PagedListBuilder</code> what I last saw and what is being displayed differs by quite a bit.</p>
  
  <pre><code>    val dataSourceFactory = dao.reportsDataSourceFactory()
      val builder = RxPagedListBuilder(
          dataSourceFactory,
          PagedList.Config.Builder()
              .setEnablePlaceholders(false)
              .setInitialLoadSizeHint(60)
              .setPageSize(20)
              .setPrefetchDistance(60)
              .build()
      )
          .setInitialLoadKey(initialLoadKey)
          .setBoundaryCallback(boundaryCallback)
  </code></pre>
  
  <p>If I'm in the <a href="https://i.stack.imgur.com/Ae3DR.png" rel="noreferrer">middle of page #4</a> when resuming - adapter will be at position at the <a href="https://i.stack.imgur.com/YkARq.png" rel="noreferrer">beginning of page #4</a>. Ideally adapter should be restored in exactly the same position as last seen.</p>
  
  <p>Various attempts to save LayoutManager state  </p>
  
  <pre><code>outState.putParcelable("layout_manager_state", recycler_view.layoutManager.onSaveInstanceState()) 
  </code></pre>
  
  <p>and then restore it  </p>
  
  <pre><code>recycler_view.layoutManager.onRestoreInstanceState(it.getParcelable("layout_manager_state"))
  </code></pre>
  
  <p>failed miserably. Any suggestions are welcome :)</p>