Keyboarding: Remove KeyboardHelper

Use functionality of keyboarding library instead.

Change-Id: Ic2228b22dde098240a0ea82b50ab487325645d3f
Keyboarding: Remove IIMEKeyboardSwitcher interface

This interface is no longer needed. Also LgKeymanHandler is now
only used on Windows, so it got excluded on Linux.

Change-Id: Ia73d5b72dc101104caa35a5a77589eb09ff27442