Android - Animate startX propery of gradient color in xml

Question

I have an animated-vector that has gradient as its stripeColor. I would like to animate the startX property of the gradient, created color_to_animate to do this, but this doesn't seem to work, am I missing something here?

Note animating other properties, e.g the strokeWidth of border_path works fine

<animated-vector xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:aapt="http://schemas.android.com/aapt">
    <aapt:attr name="android:drawable">
        <vector
            android:name="vector"
            android:width="180dp"
            android:height="120dp"
            android:viewportWidth="300"
            android:viewportHeight="200">
            <path
                android:name="border_path"
                android:pathData="
                    M 2.5,20
                    L 2.5,7.5
                    a 5,5,0,0,1,5,-5
                    L 292.5,2.5
                    a 5,5,0,0,1,5,5
                    L 297.5,192.5
                    a 5,5,0,0,1,-5,5
                    L 7.5,197.5
                    a 5,5,0,0,1,-5,-5
                    Z"
                android:strokeWidth="5">
                <aapt:attr name="android:strokeColor">
                    <gradient
                        android:name="color_to_animate"
                        android:startColor="@color/viewfinder_border_center"
                        android:endColor="#FFFFFF"
                        android:startX="0"
                        android:endX="300"
                        android:startY="0"
                        android:endY="200"
                        android:type="linear" />
                </aapt:attr>

            </path>
        </vector>
    </aapt:attr>

    <target android:name="color_to_animate">
        <aapt:attr name="android:animation">
            <objectAnimator
                android:propertyName="startX"
                android:duration="1500"
                android:valueFrom="0"
                android:valueTo="300"
                android:valueType="intType"
                android:interpolator="@android:interpolator/linear" />
        </aapt:attr>
    </target>
</animated-vector>