SF: omit vsync callbacks when screen is OFF

This CL is submitted behind a flag and in still in development.
The flag should not be enabled just yet.

Test: manual
Bug: 331636736
Flag: com.android.graphics.surfaceflinger.flags.no_vsyncs_on_screen_off
Change-Id: Iec9cde3bafb9c112c9ce9beebe6dded4727fc99c