Check magnification background type before setting corner radius.

Wear OS reuses the same FullscreenMagnificationController class, but it
has a custom layout which doesn't use the same drawable type. This
change adds a type check to prevent runtime crash in Wear OS.

Bug: 362622132
Fix: 362622132
Flag: com.android.systemui.update_corner_radius_on_display_changed
Test: manual test on watch
Change-Id: I3d8898342f4961731cb985b0209e9b0d0c2fa786