Fix rounded corners in transitions on non-internal displays

Currently getWindowCornerRaduis always returns the default display's
rounded corners because this is what is returned by
RoundedCorners#getRoundedCornerRadius etc.

This should only be done for displays that have their corner radii
in the resources, i.e. built-in internal displays - this is what
DisplayPolicy#getWindowCornerRadius checks and returns 0 otherwise.

Adding this check directly to ScreenDecorationsUtils.

Fix: 365846717
Test: presubmit
Flag: EXEMPT trivial bugfix
Change-Id: I2133917df504af1467db706918db2b6050b5067c