Respect session replay feature flag after /config endpoint call

Description

Ticket: https://posthoghelp.zendesk.com/agent/tickets/20809 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/29952)

Session recordings start as soon as the app launches, but feature flag evaluation is executed asynchronously.
If the cached value is different that the value returned from /decide endpoint, then the recording may start (and not stop) when the flag evaluates to false

A possible fix is to mem-cache screenshots and skip processing until the /decide API has been successfully called - there are some memory footprint consideration here. We may need to use a different disk-backed queue, and drop or merge the queues once the flag evaluates?

• iOS (https://github.com/PostHog/posthog-ios/issues/263)
• Android (https://github.com/PostHog/posthog-android/issues/575)
• Flutter (this issue)
• RN (https://github.com/PostHog/posthog-js/issues/3859)

Trace the Flutter session recording startup path alongside the asynchronous feature-flag evaluation and the /decide endpoint call. Review the proposed screenshot caching or queue behavior, then verify that recording does not start or continue when the evaluated session replay flag is false.