Virtual Keyboard: Initial DPI scaling tweaks

* Set preferred keyboard size to 720
  (As we're dealing with DPI-scaled maxWidth and maxHeight now)
* Explicitly define TabletKeyboard 'M' size
  (As we're using 4x bigger pixmaps for UI scaling)

Open-WebOS-DCO-1.0-Signed-Off-By: Josh Palmer <jpalmerwatkins@me.com>