placer au centre de l'écrant linear layout
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="@color/colorPrimaryDark"
android:elevation="4dp"
android:theme="@style/ThemeOverlay.AppCompat.ActionBar" />
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<Button
android:id="@+id/button2"
android:layout_width="200dp"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:background="@color/colorPrimaryDark"
android:text="text"
android:textSize="22dp" />
</RelativeLayout>
</LinearLayout>