Fix NaN in ScreenshotAnimationController

If the animation is started before the view is attached, there's no
target position for the screenshot preview (empty Rect) which causes a
NaN. This shouldn't happen, but has occasionally been observed; switch
to just skipping the animation and logging an error for that case.

Bug: 418645299
Flag: EXEMPT bugfix
Test: force the targetPosition rect to be empty and observe animation
Change-Id: Ifc17b4c249230e9c2c3defa0e843fd1357ee9e6d