问带有圆角的Android CardView显示灰色角

EN

<?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"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@android:color/transparent">

    <android.support.v7.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:cardBackgroundColor="@android:color/white"
        app:cardCornerRadius="6dp"
        app:cardElevation="5dp">

        <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="@android:color/transparent">

            <TextView
                android:id="@+id/tvProgress"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_alignParentLeft="true"
                android:layout_alignParentStart="true"
                android:layout_centerVertical="true"
                android:layout_toLeftOf="@+id/ivIcon"
                android:layout_toStartOf="@+id/ivIcon"
                android:background="@android:color/transparent"
                android:padding="@dimen/elementPaddingSmall"
                android:text="Initial Discussion"
                android:textAppearance="?android:attr/textAppearanceMedium"
                android:textColor="@android:color/black" />

            <ImageView
                android:id="@+id/ivIcon"
                android:layout_width="50dp"
                android:layout_height="50dp"
                android:layout_alignParentEnd="true"
                android:layout_alignParentRight="true"
                android:background="@color/lightBrown"
                android:scaleType="centerInside"
                android:src="@drawable/ic_checkmark_circle" />
        </RelativeLayout>


    </android.support.v7.widget.CardView>
</RelativeLayout>