Add popupKeyboardTemplate attribute to Keyboard

This change
- introduces the popupKeyboardTemplate attribute of Keyboard to
  specify XML Keyboard file for popup mini keyboard.
- introduces the maxPopupKeyboardColumn attribute of Keyboard to
  specify the maximum column of popup mini keyboard.
- changes the content format of the popupCharacters attribute of Key.
  It now represents keyLabel, codes and keyTextOutput of each key of
  popup mini keyboard using CSV format.

Bug: 2214959
Change-Id: I539e310f7e38a049ee193de0b4ad5d7afdce37b1