Update build tools

Update build tools to version 21.1.1, Android Gradle to version 0.13.2,
and Gradle to version 2.1.
Target Android API 21

Target Android API 21: Lollipop.
Remove unnecessary toString() call
Specify layout_marginLeft for API < 11

Since layout_marginStart is only available for API >= 11, we specify
layout_marginLeft for devices with API < 11.
Fix ThreeCornerKeyboardView.KitKat style

ThreeCornerKeyboardView.KitKat should inherit from KeyboardView.KitKat
rather than IceCreamSandwich.