Fix window animation flash issue

The original logic cuts down the mShownPosition, causing 1 pixel offset,
sometimes this will cause flash.

To fix this, Use Math.round() instead.

google issue:
https://code.google.com/p/android/issues/detail?id=224185

Change-Id: I8a2fe55a2df6eaa9eda4ba78966a74ea492ab8ea