title: Why is the 'prompt' of the android spinner not shown?
body: <p>In android I define a layout as follows:</p>
  
  <pre><code>&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:orientation="vertical"
      android:columnCount="4"
      android:layout_width="match_parent"
      android:layout_height="match_parent"&gt;
  
      &lt;android.support.v7.widget.Toolbar android:id="@+id/toolbar_setting"
          android:layout_width="match_parent"
          android:layout_height="?attr/actionBarSize"
          android:background="?attr/colorPrimary"/&gt;
  
      &lt;Spinner
        android:id="@+id/settings_interval"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:prompt="@string/spinner_title"/&gt;
  
        ...
  </code></pre>
  
  <p>But some some reason the <code>android:promp</code> text for the spinner is not shown  when I start the corresponding activity. For completeness here is that activity:</p>
  
  <pre><code>public class SettingsActivity extends AppCompatActivity implements AdapterView.OnItemSelectedListener {
  
       @Override
      protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.settings);
  
           // Set toolbar, allow going back.
           Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar_setting);
           //toolbar.setDisplayHomeAsUpEnabled(true);
           //toolbar.setTitle("Settings");
           setSupportActionBar(toolbar);
           getSupportActionBar().setDisplayHomeAsUpEnabled(true);
           getSupportActionBar().setTitle("Settings");
  
           Spinner spinner = (Spinner) findViewById(R.id.settings_interval);
          // Create an ArrayAdapter using the string array and a default spinner layout
          ArrayAdapter&lt;CharSequence&gt; adapter = ArrayAdapter.createFromResource(this,
              R.array.listValues, android.R.layout.simple_spinner_item);
          // Specify the layout to use when the list of choices appears
          adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
          // Apply the adapter to the spinner
          spinner.setAdapter(adapter);
          spinner.setOnItemSelectedListener(this);    
       }
  }
  </code></pre>