I am using a drawable as a background of a TextView just to have a divider line below the text. A achivied it with this drawable-xml:
In general, I try to mess as little as possible with backgrounds unless absolutely necessary, since doing so overrides the default background colors that have states for focused, pressed, etc. I suggest just using an additional view (in a vertical LinearLayout) that is as thick as you need it to be. For example:
<View
android:background="#FF000000"
android:layout_height="2dp"
android:layout_width="fill_parent"/>