If you're meaning you want them to be pushed to the edge, pixel for pixel, you'll need to use your own 9-patch for the button background, as the default one in Android has a few extra pixels around the border of its 9-patch background. For an example of this, look at the file in your SDK folder under:
platforms/android-8/data/res/drawable-hdpi/btn_default_transparent_normal.9.png
For example, this is what I get with the default:
And with a custom 9-patch I had made earlier:
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