色や角の丸みはお好みで
button.xml
<?xml version="1.0" encoding="utf-8"?>
<ripple
xmlns:android="http://schemas.android.com/apk/res/android"
android:color="@color/colorAccent">
<item>
<shape
android:shape="rectangle">
<solid android:color="@color/colorPrimary"/>
<corners android:radius="25dp"/>
</shape>
</item>
</ripple>