Revive drop shadow of SuggestionWindow.

The drop shadow of the SuggestionWindow was accidentally gone by
I579c0cc5b7f0dd337bff54af77828b8af25b13d2.

The reason of drop shadow disappearance is setting TRANSPARENT
background to the popup window.

To revive drop shadow of SuggestionWindow, follows the way of floating
toolbar.
- Create PopupWindow and make it transparent. (already exists)
- Wrap contents with RelativeLayout and set layout_margin and elevation
  to drop the shadow into the transparent PopupWindow.

The changes in Editor is for keeping this margin during re-calculation
of the contents width and position.

Bug: 15347319

Change-Id: I5a9bcbe29400d6193eb0532a5e711a78a12383cd