Android: Dynamically adapt grid span to card_game size

In order to avoid getting stuck making a new dimension file every time a new device is found we take a known value for how large the game card will be, take the screen size, and adjust the grid accordingly.

Source/Android/app/src/main/res/values/dimens.xml

@@ -1,8 +1,6 @@
 <resources>
-    <!-- Default screen margins, per the Android Design guidelines. -->
-    <dimen name="activity_horizontal_margin">16dp</dimen>
-
     <dimen name="spacing_small">4dp</dimen>
     <dimen name="spacing_medlarge">12dp</dimen>
     <dimen name="spacing_large">16dp</dimen>
+    <dimen name="card_width">140dp</dimen>
 </resources>