Simplify the Key's on state transition

Because handling shift locked state of alphabet keyboard is done by
KeyboardSwitcher, there is no need to change the "on" state of Key
when releasing the key. On the other hand, symbol and symbol shifted
keyboard has fixed "on" state "ALT/MORE" key.

This change also renames the attribute "isModifier" to "isFunctional".

Change-Id: I082885bd2376ae26bdfc378c14add2b5d6be1d4e