Remove workaround code of placing DrawingPreviewPlacerView

This CL divides MainKeyboardView.locatePreviewPlacerView method into
two methods. One is installing DrawingPreviewPlacerView to the window
as Frontmost view. Another is telling the location of MainKeyboardView
to DrawingPreviewPlacerView. Thus we can eliminate workaround code to
deal with transient states in orientation change.

Change-Id: Ia5d8f28dfb5213d27aa218c72d838a3c3be5a527