Optimize GUI zooming for Android port

The previous fliter was too slow so we do some optimzation for it. When
zooming in, let it do as how it was. When zooming out, do a
Bitmap.createScaledBitmap and a canvas drawBitmap to get it faster.
Thanks papaloco.

Change-Id: I83f22e4909ecaa1d6dffcaa7e8636da3f199a3d2