graphics: restart HWC when SurfaceFlinger stops

Currently display stays on when SurfaceFlinger is stopped, since HWC
runs as a separate service. There's no reason for display to remain on
in this state, and can be confusing to developers.

Restarting HWC when SurfaceFlinger stops causes display to turn off,
matching expected behavior. HWC is then ready to service SurfaceFlinger
when SurfaceFlinger starts back up.

Bug: 74199279
Change-Id: Ic772c29b362b3e8b2d6bc674a0bd237440880492