title: CollapsingToolbarLayout expansion not working with RecyclerView
body: <p>The behavior of CollapsibleToolbarLayout with regards to the RecyclerView seems broken. When setting a value for </p>
  
  <p>app:layout_scrollFlags</p>
  
  <p>If you do not use "scroll|enterAlways", the Collabsible layout will never expand. It will always contract, but when you pull down, it does not expand anymore, so it just gets locked to the top.</p>
  
  <p>If you use "scroll|enterAlways", it works as expected, meaning it instantly expands the header to its full height whenever you scroll down.</p>
  
  <p>Without knowing anymore, it seems as if the Collapsible layout cannot properly determine when the RecyclerView is at the top of the list, as that is the criteria for when it should expand to full height in the other cases.</p>
  
  <p>Here is the layout that I am testing with. The java code isn't really important, it is just populating the recyclerview with dummy data and using a LinearLayoutManager.</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  
  &lt;android.support.design.widget.CoordinatorLayout
      xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      android:layout_width="match_parent"
      android:layout_height="match_parent"&gt;
  
  &lt;android.support.v7.widget.RecyclerView
      android:id="@+id/recycler_view"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      app:layout_behavior="@string/appbar_scrolling_view_behavior" /&gt;
  
  &lt;android.support.design.widget.AppBarLayout
      android:id="@+id/appbar"
      android:layout_width="match_parent"
      android:layout_height="200dp"
      android:theme="@style/ThemeOverlay.AppCompat.ActionBar"
      android:fitsSystemWindows="true"&gt;
  
      &lt;android.support.design.widget.CollapsingToolbarLayout
          android:id="@+id/collapsing_toolbar"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:fitsSystemWindows="true"
          app:toolbarId="@id/toolbar_id"
          app:contentScrim="@color/white"
          app:layout_scrollFlags="scroll|exitUntilCollapsed"&gt;
  
          &lt;android.support.v7.widget.Toolbar
              android:id="@+id/toolbar_id"
              android:layout_width="match_parent"
              android:layout_height="?android:attr/actionBarSize"
              app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
              app:layout_collapseMode="pin"/&gt;
  
      &lt;/android.support.design.widget.CollapsingToolbarLayout&gt;
  
  &lt;/android.support.design.widget.AppBarLayout&gt;
  
  &lt;/android.support.design.widget.CoordinatorLayout&gt;
  </code></pre>