Bump timeout while waiting for sandbox process to stop.

When the sandbox process has a large memory footprint (say >25 GiB), it can
take more than 5 seconds for the sandobx process to dissapear from the
process table after receiving SIGKILL.

This is more applicable for GPU applications which can use very large amounts
of memory.

PiperOrigin-RevId: 617931970

runsc/sandbox/sandbox.go

@@ -1462,7 +1462,7 @@ func (s *Sandbox) waitForStopped() error {
 		return nil
 	}
 
-	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
 	defer cancel()
 	b := backoff.WithContext(backoff.NewConstantBackOff(100*time.Millisecond), ctx)
 	op := func() error {