Device manager - last_seen_user_agent device property

[kerryarchibald]

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.

[SimonBrandner]

Suggested change

	last_seen_user_agent?: string;
	"org.matrix.msc3852.last_seen_user_agent"?: string;
	[UNSTABLE_MSC3852_LAST_SEEN_UA.stable]?: string;
	[UNSTABLE_MSC3852_LAST_SEEN_UA.unstable]?: string;