Gallery2: fix display abnormal when tap sharpness in filtershow

In Android4.3 version, it using stride to pad row layouts in
script, and can't do 1D indexing of 2D objects, this make
the resulting pixel values are not the corresponding
coordinates' pixel value.

We change to use rsGetElementAt() not gPixels() to
get the correct pixel values.

CRs-Fixed: 539928

Change-Id: Ifc0e516014c9fd974f3e7df163b06f732499f7c0