Do not use the status bar as the system decor layer if it is not visible.

Doing so leads to cropping of the content view, which is smaller than the
screen size on some devices with improperly configured overscan values. It
can also lead to performance issues with some activity transition
animations on Wear.

Bug: 19928768
Change-Id: Ic50c2fedb1f8be9a8ea5166fef5a5665958601de