title: back button not showing up in CollapsingToolbarLayout
body: <p>Hi I am doing a simple app to try out material design. Unfortunately, following the incredible tutorial <a href="https://guides.codepath.com/android/Handling-Scrolls-with-CoordinatorLayout#expanding-and-collapsing-toolbars" rel="noreferrer">here</a> and trying out the samples <a href="https://github.com/chrisbanes/cheesesquare" rel="noreferrer">here</a> (These are incredible sources for FYI in material design) I am struggling to make my app toolbar to work perfectly as it should. For instance, the back button right next to the title on collapse in not showing up. BUT I can simply click that area where it should be and it responded. </p>
  
  <p>So therefore the button is there but it is not showing up and I cannot figure out why. Also, as the toolbar collapsed, the toolbar doesn't change its color to primary color I set. contentScrim doesn't seem to react on collapse mode but I will post another question for this.</p>
  
  <p>Here is what it currently looks like:</p>
  
  <p><a href="https://i.stack.imgur.com/JiSYn.png" rel="noreferrer"><img src="https://i.stack.imgur.com/JiSYn.png" alt="enter image description here"></a></p>
  
  <p>Now on collapse, the title is noticeably far to the right. The button is there, I can click it, it returns to my app's homescreen but the back icon is not there.</p>
  
  <p><a href="https://i.stack.imgur.com/K9rJ2.png" rel="noreferrer"><img src="https://i.stack.imgur.com/K9rJ2.png" alt="enter image description here"></a></p>
  
  <p>Here is the layout file for the activity:</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"
      xmlns:tools="http://schemas.android.com/tools"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:fitsSystemWindows="true"
      android:orientation="vertical"
      tools:context=".HomeScreenActivity"
      android:weightSum="1"&gt;
  
      &lt;android.support.design.widget.AppBarLayout
          android:id="@+id/appbar_note"
          android:layout_width="match_parent"
          android:layout_height="168dp"
          android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
          android:fitsSystemWindows="true"&gt;
  
          &lt;android.support.design.widget.CollapsingToolbarLayout
              android:id="@+id/collapsingtoolbarlayout_note"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              android:fitsSystemWindows="true"
              app:expandedTitleMarginEnd="64dp"
              app:expandedTitleMarginStart="48dp"
              app:contentScrim="?attr/colorPrimary"
              app:layout_scrollFlags="scroll|exitUntilCollapsed"&gt;
  
              &lt;android.support.v7.widget.Toolbar
                  android:id="@+id/toolbar_note"
                  android:layout_width="match_parent"
                  android:layout_height="?attr/actionBarSize"
                  app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
                  app:layout_collapseMode="pin" /&gt;
  
              &lt;ImageView
                  android:src="@drawable/backgroud_note"
                  android:layout_width="match_parent"
                  android:layout_height="match_parent"
                  android:scaleType="centerCrop"
                  android:fitsSystemWindows="true"
                  app:layout_scrollFlags="scroll|enterAlways|enterAlwaysCollapsed"
                  app:layout_collapseMode="parallax"/&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="fill_parent"
          android:fillViewport="true"
          app:layout_behavior="@string/appbar_scrolling_view_behavior"&gt;
          ...    
      &lt;/android.support.v4.widget.NestedScrollView&gt;
  
      &lt;android.support.design.widget.FloatingActionButton
          android:id="@+id/fab_note"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          app:layout_anchor="@id/appbar_note"
          app:layout_anchorGravity="bottom|right|end"
          android:layout_margin="@dimen/fab_margin"
          android:src="@drawable/ic_send_white_24dp" /&gt;
  
  &lt;/android.support.design.widget.CoordinatorLayout&gt;
  </code></pre>
  
  <p>I am using API 22 build 23.0.2 all of my support libraries are at 23.1.0</p>