Fix crash in download_controller

# Issue
Inside DownloadAppVerificationRequest constructor, we call StartIsVerifyAppsEnabled directly, which may cause the callback to be called before the constructor finished.

So for the code like this:
```
  app_verification_requests_[item] =
      std::make_unique<DownloadAppVerificationRequest>(
          item, base::BindOnce(&DownloadController::OnAppVerificationComplete,
                               // Unretained is safe because this is a singleton
                               base::Unretained(this)));
```
The callback `OnAppVerificationComplete` may be called, before putted into app_verification_requests_, that may cause crash as we check the item existance in app_verification_requests_ inside callback `OnAppVerificationComplete`.

And the callback allows only called once, which we need to add return after callback is run.

Change-Id: I3b56c4daadf58c7e4803c01ea0a638e4d1b22d57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5652144
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1319872}