Revert "Remove circular mask"

This reverts commit 2e8b27e8.

It also makes the circularMask option configurable via overlay.
It is set to false by default.

An offset dimension entry is also provided, for displays which have
a "chin". This is set to 0 by default.

Bug: 16380982
Change-Id: I16ec3e8d8882a6683c9155055944e5c2f5cd781e