DevTools handle --start-paused in profile mode

[terrylucas]

flutter run --start-paused

Attaching to a running flutter app started with '--start-paused'. There is no way to resume the app because the debugger isn't available.

Add a dialog, if this happens, that says resume paused application.

[devoncarew]

We may be able to combine this with the work to show the paused state on non-debugger pages. For that I was think of a small, in-line dialog or UI element that would have a resume button (and potentially one or two other actions?).