Ensure starting position rect isn't empty in FloatingTaskView

* on rotation taskbar re-inflates it's hotseat views.
If we're already in split, then FloatingTaskView tries to do calculations
on the view's position in the window, but has a reference to the
old, pre-rotated view which is no longer in any window.
* This sets startingPosition to be an empty rect, which is used to
set LayoutParam values, which are used downstream in update() to calculate
scale, and thus we end up dividing by 0.
* TODO(b/345556328) figure out a better solution than checking for an
empty rect

Fixes: 342606096
Test: Repro steps don't cause crash
Flag: EXEMPT bugfix
Change-Id: Icb546a05d383d1997a92471fc1de3ffc37d06eca