Well there is no general solution for that, you have to remember there is no universal keyboard and even if there were people can just download a different keyboard app from the Play Store. Samsung, HTC, Nexus, all have there own keyboard and many oem even have different ones for different phones.
So aside from setting:
android:inputType="number"
on your EditText's there is nothing you can do. Every keyboard handles this in there own way.
EDIT:
If you control which software is installed than you can write a custom keyboard app or search for a keyboard app which supports the feature you need and which you can ship with your tablet. But aside from that there is nothing you can do.
EDIT 2:
Here is a link to a tutorial which explains how to write a custom keyboard: link
EDIT 3:
You always have the option of just emulating the keyboard. Place a fragment containing the buttons you need at the bottom of the screen with visibility View.GONE
and set it to View.VISIBLE
when the user needs it. You can prevent the normal keyboard from appearing with this code:
InputMethodManager imm = (InputMethodManager)getSystemService(Context.INPUT_METHOD_SERVICE);
imm.hideSoftInputFromWindow(myEditText.getWindowToken(), 0);
I actually did something like that for an app I wrote a few months ago. When you apply a fade & translation animation to the fragment as you set it to View.VISIBLE
it actually looks pretty good!