[android] Change Spinner dropdown icon

Add theme to spinner

<Spinner style="@style/SpinnerTheme"
      android:layout_width="match_parent"
      android:layout_height="match_parent"/>

Add spinnerTheme to styles.xml

<style name="SpinnerTheme" parent="android:Widget.Spinner">
    <item name="android:background">@drawable/spinner_background</item>
</style>

Add New -> "Vector Asset" to drawable with e.g. "ic_keyboard_arrow_down_24dp"

Add spinner_background.xml to drawable

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <layer-list>
            <item android:drawable="@drawable/ic_keyboard_arrow_down_24dp" android:gravity="center_vertical|right" android:right="5dp"/>
        </layer-list>
    </item>
</selector>