Inform SurfaceFlinger of which Surface is the RoundedCorners overlay.

We need to omit it from screenshots, and screen mirroring, so SurfaceFlinger
will have to have some knowledge. Eventually we'd like to do this by
having everything else have a parent, and mirroring and screenshotting will
operate on that parent. Since we aren't comfortable doing that for DR though, here's
a quick and dirty hack to tide us over with low risk.

Bug: 63311708
Test: screencap, screenrecord, android.view.cts.SurfaceViewSyncTest
Change-Id: I2b43be00567ee19cf27aeef344e5415f4a69297f