Add SurfaceFlingerStuffing case to toString()

After the recent addition of this jank type, toString() was missed in
teh change resulting in a crash.

Bug: 189356168
Test: android.view.cts.AttachedSurfaceControlSyncTest#testSync
Change-Id: I82496a8bf95cdd1304c0c17a1c9c5d38416a4a88