perf(android): Defer SentryFrameMetricsCollector thread startup (#5641)

* perf(android): Start frame metrics thread lazily on first collection

SentryFrameMetricsCollector created and started its HandlerThread in the
constructor, blocking the calling thread (the main thread during SDK
init) on HandlerThread.getLooper(). The handler is only needed once
startCollection() registers a listener, so start the thread lazily there
instead. Apps that never collect frame metrics no longer start the
thread at all.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* changelog

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>