Manage Account: declare sessionToken scope during FxA initialization

[grigoryk]

We need a new sessionToken scope in order to enable "Manage Account" functionality in Fenix's Account Settings.

For existing FxA users, introduction of this scope will be a non-issue, until they will need to interact with upcoming "Manage Account" functionality (a button, which opens up FxA web-content and passes it the sessionToken via webchannel). Once that happens, we will need to trigger a "need to re-authenticate" state. This is unfortunate, but the alternative is worse - triggering that state for everyone upon introduction of this scope. See a-c where this work is tracked: mozilla-mobile/android-components#4703

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