Remove @UnsupportedAppUsage from DreamService.setWindowless

setWindowless is a hidden API since 2014 and was marked
@UnsupportedAppUsage in 2018. It is not known to be used anywhere as
reported by go/nonsdk-dash. This CL removes the @UnsupportedAppUsage.

Bug: 151221085

Test: m

Change-Id: Ic95a2d9da4eb9f4abeba75a35ebe1f8921fb2416