如果想要在LinearLayout中设置居中，需要用到gravity
android:gravity: takes care of its children
android:layout_gravity: takes care of itself.
<LinearLayout xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:gravity="center_horizontal"
tools:context="com.example.android.jlapp.MainActivity"/>
