Try something like this
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:innerRadius="0dp"
android:shape="ring"
android:thicknessRatio="2"
android:useLevel="false" >
<solid android:color="@android:color/transparent" />
<stroke
android:width="2dp"
android:color="@android:color/darker_gray" />
</shape>
Update: made android:thicknessRatio="2"
to give full circle (using Nexus 5 – Lollipop)