title: Change toolbar colour on scroll without collapsing
body: <p>I'm trying to make my toolbar change colour on scroll without actually collapsing it (so I don't want the height of it to change). I want it to be one colour initially and then it will change to another when you scroll. It should then change back to the original colour when I scroll back to the top.</p>
  
  <p>I've been able to change the colour on scroll but only in conjunction with the height changing. I've tried setting minHeight on the Toolbar, CollapsingToolbarLayout and AppBarLayout, but no luck unfortunately.</p>
  
  <p>I know that I can probably do this manually (animate the colour on scroll change) but I want to rule out doing it with the design library first.</p>
  
  <p>This is what I have so far:</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:tools="http://schemas.android.com/tools"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      android:background="@color/app_secondary_darker"&gt;
  
      &lt;android.support.design.widget.AppBarLayout
          xmlns:android="http://schemas.android.com/apk/res/android"
          xmlns:app="http://schemas.android.com/apk/res-auto"
          android:id="@+id/appbar"
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:minHeight="?attr/actionBarSize"
          android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
          android:fitsSystemWindows="true"&gt;
  
          &lt;android.support.design.widget.CollapsingToolbarLayout
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:minHeight="?attr/actionBarSize"
              android:fitsSystemWindows="true"
              app:contentScrim="@color/white"
              app:layout_scrollFlags="scroll"&gt;
  
              &lt;android.support.v7.widget.Toolbar
                  android:id="@+id/toolbar"
                  android:layout_width="match_parent"
                  android:layout_height="?attr/actionBarSize"
                  android:minHeight="?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.v4.widget.NestedScrollView
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          app:layout_behavior="@string/appbar_scrolling_view_behavior"&gt;
  
          &lt;LinearLayout
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:orientation="vertical"&gt;
  
              &lt;TextView
                  android:layout_width="match_parent"
                  android:layout_height="400dp"
                  android:text="Abc"
                  android:background="@color/grey_light"/&gt;
  
              &lt;TextView
                  android:layout_width="match_parent"
                  android:layout_height="400dp"
                  android:text="Def"
                  android:background="@color/red_dark"/&gt;
  
          &lt;/LinearLayout&gt;
  
      &lt;/android.support.v4.widget.NestedScrollView&gt;
  
  &lt;/android.support.design.widget.CoordinatorLayout&gt;
  </code></pre>
  
  <p>Thanks in advance!</p>