google-material-design-icons/android/action/maximize/materialiconsround/black/res/drawable/round_maximize_20.xml

<vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:width="20dp"
    android:height="20dp"
    android:viewportWidth="20"
    android:viewportHeight="20"
    android:tint="?attr/colorControlNormal">
  <path
      android:fillColor="@android:color/white"
      android:pathData="M3.75,3h12.5C16.66,3 17,3.34 17,3.75v0c0,0.41 -0.34,0.75 -0.75,0.75H3.75C3.34,4.5 3,4.16 3,3.75v0C3,3.34 3.34,3 3.75,3z"/>
</vector>