Fix 6656710: center dot pattern in GlowPadView

This fixes a bug where the dots weren't aligned with the center of the view.
The matrix should not contain the offset of the image itself since we really
want to scale about the abstract points.  The code now subtracts the image
centering positions (cx,cy) from the point to be scaled.

Change-Id: I1d46a5d51b1fd1d39944c64baf86c306c2d5e923