Add KeyEvent handler to LauncherClings.

When the longpress clings such like welcome message appears,
the key events should be limited as follows:
- normal keys (generates printable glyphs) should be prevented,
  otherwise they come to the underlying searchbox.
- it should be dismissed on Menu keys, which goes to the overview
  mode.

Bug: 22982955
Change-Id: I7272b0809bfaa3c08ba77b01383cf54f19390d95