I know it is possible to make a shape looking something like this:
But I don\'
You can define it in xml in a shapes file, but it may be a lot easier to make a simple 9 patch graphic, then you can easily customise how and where the curved and straight segments will be stretched.
See http://developer.android.com/tools/help/draw9patch.html for more info
EDIT
For more info on shapes see here: http://developer.android.com/guide/topics/resources/drawable-resource.html#Shape