RUM-8888 Auto-stop manually tracked views on app background

[barboraplasovska]

What and why?

• Manually tracked RUM views no longer keep running when the app backgrounds. They're now automatically stopped on background and restarted with the same identity on foreground, matching how automatic UIKit/SwiftUI views already behave. Fixes RUM-8888.
• Adds a no-arg stopView() overload to RUMMonitorProtocol so customers can explicitly stop whatever view is currently active.
• Adds a new trackViewsAutomaticStopOnBackground config flag (default true, opt-out) for customers who rely on the current behavior. Defaulting to true is a deliberate breaking-change decision.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes
• Add Objective-C interface for public APIs - see our guidelines (internal)
• Run make api-surface when adding new APIs