Correct the behavior of "Desktop Only" in Notifications preferences

Summary:
See https://discourse.phabricator-community.org/t/desktop-only-notifications-mode-is-broken/1234. Ref T13102. The "Desktop Only" mode for notifications currently shows both desktop and web notifications.

In fact, JX.Notification currently has no ability to render notifications as desktop-only. Make this work.

Note that many of the variables and parameters here, including showAnyNotification, web_ready, and desktop_ready, are named in an incorrect or misleading way. However, the new behavior appears to be correct.

Test Plan:

• Emitted test notifications in "No Notifications", "Web Only", "Web and Desktop", and "Desktop" modes.
• Saw appropriate notifications appear in the UI.

Maniphest Tasks: T13102

Differential Revision: https://secure.phabricator.com/D19233