Reset docked divider to the middle of the screen if sys-ui dies

It is possible for the docked divider to be in an unstable location
when sys-ui dies. E.g sys-ui animating the divider from behind the
nav bar. When this occurs we reset the divider to the middle of the
screen so that the system remains in a useable state. Otherwise, the
docked stack can be fullscreen always preventing the user from going
into any other application.

Bug: 26965731
Change-Id: Icd254fffe69da4ee3f2efb4ff1d210a778703f64