Don't sync pinned stack bounds due to onDown unless we're not animating.

This was causing PIP to be offset from the touch position if it was dragged before its animation ended.

This also fixes a bug where mAnimatingToBounds was actually empty during animations, because we were calling cancelAnimations() after setting it.

Test: fling pip, then grab it immediately and drag it
Bug: 153094531
Change-Id: Ic51e1a79e597a0cbe4b0d3e7a238f8c92d8712c0