Add tablet detection for the "desktop mode by default" change for Firefox on Android tablets

[ksy36]

As https://bugzilla.mozilla.org/show_bug.cgi?id=1910768 shipping, Firefox on Android tablets have desktop mode turned on by default, which means they’ll have Linux User Agent string.

This ticket introduces desktop browsing by default for tablets and other large devices. It is live in Nightly and is intended to ride the trains to Release.
Desktop mode browsing by default for large devices.
When users boot up Fenix for the first time after this change, they will have desktop mode by default turned ON if they have a large screen size.
Users of Fenix on tablets or Chromebooks will now have desktop browsing defaulted to ON for all newly-opened tabs.

There have been a number of issues coming from tablets with browser-fenix+os-linux https://github.com/webcompat/web-bugs/issues?q=is%3Aissue+label%3Abrowser-fenix+label%3Aos-linux+ as this has been shipping in Nightly for some time.

Example issue:
webcompat/web-bugs#143431 (UA: Mozilla/5.0 (X11; Linux x86_64; rv:134.0) Gecko/20100101 Firefox/134.0 + extra_labels: browser-fenix )

We can automatically add a label device-tablet , if an issue is reported with Linux UA and browser-fenix in extra_labels