Add new experiment for fixed UA logic

[jaceklyp]

Task/Issue URL: https://app.asana.com/0/1203790657351911/1205652551515177/f

CC: @jonathanKingston

Description:

Steps to test this PR:

1. Make sure UA tests pass
2. Install the new app and make sure you've been assigned with "mf" variant.
3. Open any page and see if the fixed user agent is being used - the one with static WebView and Safari versions.

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Internal references:

Software Engineering Expectations
Technical Design Template

[jonathanKingston]

Logic looks correct thank you, get someone who knows the feature flagging and variants to validate that's correct though.

[brindy]

LGTM!