[Bundle][FrameworkBundle] make the stopwatch service always available

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11347
| License       | MIT
| Doc PR        | 

Previously, one had to be careful to check if the ``debug.stopwatch`` service was available before using it. Otherwise, the application would break in the prod environment.
Thanks for noticing. Though keeping the check for ``$this->stopwatch !== null`` is mandatory, isn't it?