[SupportedBrowsers] Add support for Mull

[SkewedZeppelin]

Mull is a hardened FOSS fork of Fenix.

Legacy support is also provided in the unlikely event any users are
running the legacy Fennec-based Mull.

Resolves:
Divested-Mobile/Mull-Fenix#1
https://community.bitwarden.com/t/auto-fill-support-for-mull-browser-secure-firefox-android-fork/27454

Signed-off-by: Tad tad@spotco.us

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[hbarsaiyan]

@SkewedZeppelin you have not signed the CLA

[SkewedZeppelin]

@hbarsaiyan
I don't think a binding legal agreement is required for 3 lines of constants.
If that is a hard requirement for this repository, this can be closed.

[hbarsaiyan]

@SkewedZeppelin I also don't have any idea about this. I came to create a PR for Mull but you had already created one and the above comment says that "we ask that you sign our Contributor License Agreement before we can accept your contribution." so i thought you missed it by chance.