Improve the circularMask for round displays

The mask was earlier drawn as a thin ring at the display periphery.
This had undesirable effects when screenshots were taken, as the ring
was seen, as well as display content beyond it.

This patch modifies the mask to be a black canvas with portions erased
to improve the screenshot image.

Bug: 18772987
Change-Id: I25ef6387879613354308e015446fe325ed8c4515