Fix context menu for empty directory/root view.

Since it's not always true that there is a doc/directory given the (x,
y) of the right click, I've changed the signature of right click to take
in a MotionEvent object as a parameter, and make a MotionInputEvent
inside the function as a check whether it's on a file/doc or on
empty/blank pane.

Bug: 29538570
Change-Id: I7ee960c9f556752bf314ab8cc2d94fda28194c0a