Don't clear timers when runApp returns

The lifetimes of reactive expressions and observers can span multiple runApp calls, so timers should as well.

```
library(shiny)
now <- reactive({invalidateLater(1000, NULL); Sys.time()})
shiny::runApp(list(
  ui=basicPage(textOutput('foo')),
  server = function(input, output) {
    output$foo <- renderText({now()})
  }
))
```

Then stop the app and re-run the runApp command. Without this fix, the time doesn't change anymore for the second and subsequent times.