I've recreated the behavior of original bottom line. It includes paddings on the edges and same behavior when focused.
expression_edit_text_bgc_selector.xml
(set it in android:background)
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_focused="true"
android:drawable="@drawable/expression_edit_text_bgc_focused"/>
<item android:drawable="@drawable/expression_edit_text_bgc_normal"/>
</selector>
expression_edit_text_bgc_focused.xml
<?xml version="1.0" encoding="utf-8"?>
<layer-list
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools">
<!--<item android:drawable="@color/main_bg_color"/>-->
<item>
<shape>
<solid android:color="@android:color/transparent"/>
</shape>
</item>
<item
android:gravity="bottom"
android:bottom="8dp"
android:left="4dp"
android:right="4dp">
<shape android:shape="rectangle">
<size android:height="2dp"/>
<solid android:color="?colorAccent"/>
</shape>
</item>
</layer-list>
expression_edit_text_bgc_normal.xml
<?xml version="1.0" encoding="utf-8"?>
<layer-list
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools">
<!--<item android:drawable="@color/main_bg_color"/>-->
<item>
<shape>
<solid android:color="@android:color/transparent"/>
</shape>
</item>
<item
android:gravity="bottom"
android:bottom="8dp"
android:left="4dp"
android:right="4dp">
<shape android:shape="rectangle">
<size android:height="1dp"/>
<solid android:color="?android:colorControlNormal"/>
</shape>
</item>
</layer-list>