session-android/res/drawable/send_button_background.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <!-- pressed -->
    <item android:state_pressed="true" >
        <shape>
            <solid android:color="#FF33B5E5" />
        </shape>
    </item>

    <!-- focused -->
    <item android:state_focused="true" >
        <shape>
            <solid android:color="#FF33B5E5" />
        </shape>
    </item>

    <!-- default -->
    <item>
        <shape>
            <solid android:color="#00ffffff" />
        </shape>
    </item>

</selector>