Try changing "c.drawColor(0);" to "c.drawColor(0xFF000000);"
I think that using "0" for your color has an alpha of 0, so you are redrawing the background with a totally transparent black color, which basically does nothing. In what I suggested, the hex value 0xFF000000 will use a fully opaque color of black.
to see more about the Android Color object.
Let me know if this works for you.