Improve Filterable implementation for User Agents

[Dav1dde]

The Filterable::user_agent implementation may already have access to a parsed variant of the user agent, e.g. it may already exist in the browser context. Instead of relying to re-parse the user agent, it could be constructed from the context instead (if available).

[linear[bot]]

RELAY-147 Improve Filterable implementation for User Agents