I know this question is old, but here is my solution. I could not add a gradient to the stroke of the oval. The gradient always went to the center of the circle. The hack around this was to create a list of layers and create 5 concentric circles, each 1pd smaller, which added their own shadding.
, ImageView, 180x180, # 1A000000 # 00000000, :
<?xml version="1.0" encoding="utf-8"?>
<layer-list
xmlns:android="http://schemas.android.com/apk/res/android">
<item>
<shape
android:shape="oval">
<stroke android:width="1dp"
android:color="#00000000" />
<size
android:width="190dp"
android:height="190dp"/>
</shape>
</item>
<item
android:top="1dp"
android:right="1dp"
android:bottom="1dp"
android:left="1dp">
<shape
android:shape="oval">
<stroke android:width="1dp"
android:color="#05000000" />
<size
android:width="190dp"
android:height="190dp"/>
</shape>
</item>
<item
android:top="2dp"
android:right="2dp"
android:bottom="2dp"
android:left="2dp">
<shape
android:shape="oval">
<stroke android:width="1dp"
android:color="#0A000000" />
<size
android:width="190dp"
android:height="190dp"/>
</shape>
</item>
<item
android:top="3dp"
android:right="3dp"
android:bottom="3dp"
android:left="3dp">
<shape
android:shape="oval">
<stroke android:width="1dp"
android:color="#0F000000" />
<size
android:width="190dp"
android:height="190dp"/>
</shape>
</item>
<item
android:top="4dp"
android:right="4dp"
android:bottom="4dp"
android:left="4dp">
<shape
android:shape="oval">
<stroke android:width="1dp"
android:color="#15000000" />
<size
android:width="190dp"
android:height="190dp"/>
</shape>
</item>
<item
android:top="5dp"
android:right="5dp"
android:bottom="5dp"
android:left="5dp">
<shape
android:shape="oval">
<stroke android:width="1dp"
android:color="#1A000000" />
<size
android:width="190dp"
android:height="190dp"/>
</shape>
</item>
</layer-list>