Fix override insets when dragging divider in split-screen

Previously, if frame extended beyond screen, override inset was set
to zero for corresponding dimension. This caused issues while dragging
down in vertical split-screen because navigation bar inset was not
included.
This CL sets override inset value as a difference between corresponding
content area dimension and screen dimension.

Bug: 27970692
Change-Id: I5bd16077a7deb039516bc9e11aa58315f809455a