manifest.xml

<manifest xmlns:android="http://schemas.android.com/apk/res/android">
  <application
      android:allowBackup="false"
      android:fullBackupContent="false"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/playground_activity">
    <activity android:name="PlaygroundActivity"
              android:exported="true" <!--この行を新規追加 -->
              android:theme="@android:style/Theme.NoTitleBar.Fullscreen"
              android:launchMode="singleTask"
              android:configChanges="orientation|keyboardHidden|screenSize">