how to remove left margin of Android Toolbar?

replace your xml with below xml

<android.support.v7.widget.Toolbar
    android:id="@+id/toolbar"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:layout_alignParentTop="true"
    android:background="?attr/colorPrimary"
    android:elevation="@dimen/margin_padding_8dp"
    android:contentInsetStart="0dp" 
    android:contentInsetLeft="0dp"
    android:contentInsetRight="0dp"
    android:contentInsetEnd="0dp"
    app:contentInsetLeft="0dp"
    app:contentInsetStart="0dp"
    app:contentInsetRight="0dp"
    app:contentInsetEnd="0dp">

<RelativeLayout
    android:id="@+id/rlToolbar"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/tvTitle"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:paddingRight="@dimen/margin_padding_16dp"
        android:text="AppBar"
        android:textAppearance="@style/TextAppearance.AppCompat"
        android:textColor="@color/white"
        android:textSize="@dimen/text_size_20sp" />

</RelativeLayout>