content_main.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="info.androidhive.dexterpermissions.MainActivity"
    tools:showIn="@layout/activity_main">

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="100dp"
        android:orientation="vertical"
        android:layout_centerHorizontal="true"
        android:paddingLeft="16dp"
        android:paddingRight="16dp">

        <Button
            android:id="@+id/btn_camera"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="16dp"
            android:text="CAMERA PERMISSION" />

        <Button
            android:id="@+id/btn_storage"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="MULTIPLE PERMISSIONS" />

    </LinearLayout>
</RelativeLayout>