Before this commit the `languageinheader` query param would only take
effect if the A/B test was enabled AND the query param was set. Per
T282543, we want the query param to take effect regardless of the state
of the language/AB test config.
To see new treatment, set `languageinheader=1`.
To see old treatment, set `languageinheader=0`.
Bug: T282543
Change-Id: I6a06e90b6e46a6fd7506a5ddeaf071b893ebfe8e
* Adds ab test config to enable/disable the ab test. Defaults to `false`
(ab test disabled).
* Adds a `languageinheader` query param which only takes effect when the
ab test is enabled. The query param is cast to a bool and determines
which treatment is shown. For example, set query param to
`languageinheader=1` to see the new treatment. Set query param to
`languageinheader=0` to see the old treatment. To bucket based on the
user's id or global user's id, don't set the query param.
* Moves the language in header config work that was previously in
ServiceWiring into a `LanguageInHeaderTreatmentRequirement` class so
that unit tests can be done on most of the logic that determines whether
the language in header will show.
* Adds logic to bucket user based on [global] user id.
Bug: T280825
Change-Id: Id538fe6e09002fae6c371109769f3b7d61e7ac6d
