Tray doesn't indicate private icon on when restoring a private mode session

[MortimerGoro]

Also the background is not dimmed. It seems that the tray didn't receive the listener in that case.

Steps to Reproduce

1. Open FxR and switch to private mode
2. Click back and exit the browser
3. Reopen it again from the Oculus Library

Current Behavior

• Private session correctly restored
• Tray icon not enabled
• Background not dimmed

Expected Behavior

• Tray private mode icon enabled
• Background dimmed

[cvan]

A few comments to add about the status here:

• There are several issues around Private Browsing state. The key goal: make it more obvious that the user is in a Private-Browsing session. (Keep in mind that when Tabs are added after v1.1, there will be more elegant ways to address this.)
• If you test entering Fullscreen from a YouTube video, you can see a bug with the opacity (it gets lighter than the dim of Private Browsing).
• There's a PDF of the design spec for this in issue Private browsing Interaction colors are inconsistent #774: Private browsing Interaction colors are inconsistent #774 (comment)
• To whomever works on these issues: Feel free to do the work in Private browsing Interaction colors are inconsistent #774 and/or Tray doesn't indicate private icon on when restoring a private mode session #671. Or if there are follow-up issues to be filed, we can do that as well.

Thank you in advance for all the polish here! 👍