Remove unnecessary/erroneous reference counting

The SkBitmap* used by HWUI holds a reference to the PixelRef (which
in turn holds the colorTable) so keeping an additional ref is not
only unnecessary, but also potentially problematic.

If a bitmap changes its pixelRef after it has been added to a displayList,
then we end up with unbalanced ref/unref calls that cause the newly added
PixelRef to be unref'd and prematurely deleted, while the original PixelRef
is leaked.

bug: 18659976
Change-Id: I34a06152e7bb2d733249a207dafd36b9b5dc0712