Consume KEYCODE_BACK event when drawer is open

Bug: 34343480

Test: press non-arrow/tab keys when drawer is open and make sure
      search mode is not activated; press BACK button on phone or
      ESC button on physical keyboard when drawer is open and make
      sure drawer is closed.

Change-Id: Id9c584c4dff16afd9e6c50bfcad25ec6a4972105