Clear startingDisplayed flag when removing window.

The flag indicating that the Starting window is displayed was not
being cleared when the Starting window was removed. That caused the
goodToGo indication to falsely indicate that all windows were drawn
when in fact the destination activity had not yet been drawn. This
caused the animation to begin when it was still black behind the old
animation.

This fixes bug 6764727.

Change-Id: Iacef73b0335b9bde2cdc8d0b072034222cd728e8