title: My app_name is not showing when I lauch the emulator
body: <p>My main.xml is like this:</p>
  
  <pre><code>    &lt;?xml version="1.0" encoding="utf-8"?&gt;
      &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:orientation="vertical"
          android:background="@color/background"
          xmlns:tools="http://schemas.android.com/tools"
          android:paddingBottom="@dimen/activity_vertical_margin"
          android:paddingLeft="@dimen/activity_horizontal_margin"
          android:paddingRight="@dimen/activity_horizontal_margin"
          android:paddingTop="@dimen/activity_vertical_margin"
          tools:context="com.example.qiaoyi.tipcalculator.MainActivity"&gt;
  </code></pre>
  
  <p>And my Manifest looks like this:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.example.qiaoyi.tipcalculator"&gt;
  
      &lt;application
          android:allowBackup="true"
          android:icon="@mipmap/ic_launcher"
          android:label="@string/app_name"
          android:supportsRtl="true"
          android:theme="@style/AppTheme"&gt;
          &lt;activity android:name=".MainActivity"&gt;
              &lt;intent-filter&gt;
                  &lt;action android:name="android.intent.action.MAIN" /&gt;
  
                  &lt;category android:name="android.intent.category.LAUNCHER" /&gt;
              &lt;/intent-filter&gt;
          &lt;/activity&gt;
      &lt;/application&gt;
  
  &lt;/manifest&gt;
  </code></pre>
  
  <p>I tried to add android:label="@string/app_name" to my manifest before, but it still does not work. So I take it out. What is the problem with my codes? Thanks.</p>