input: introduce new navigation bar key event source

* When generating virtual navigation bar KeyEvent, set
  a specific source fort it. There is no precise way to
  recognize if the KeyEvent is coming from KeyButtonView.
  Another way is passing another flag.

Change-Id: I243c9f91afcbf4c430e2d526e8c814a5a7975e51
Signed-off-by: Mohammad Hasan Keramat J <ikeramat80@gmail.com>