Made SurfaceSyncGroup final

Because SurfaceSyncGroup is public, it needs to be final. However, VRI
was relying on overriding SurfaceSyncGroup to listen for sync ready.
Instead, just add a way for VRI to register when SSG is ready so it
doesn't need to extends SurfaceSyncGroup.

Test: SurfaceSyncGroupTest
Fixes: 267312017
Change-Id: If0475d3da90d7a3c3c76ab48a01e8bf2f15186b6