Discouraged Private API use

SentryFrameMetricsCollector uses reflection to access `mLastFrameTimeNanos` which is not part of the public SDK and might change in future releases

Start by locating SentryFrameMetricsCollector and inspect how it reflects on mLastFrameTimeNanos. Check the Android frame-metrics APIs and existing tests around frame collection. Done should mean the collector no longer relies on that non-public SDK field while preserving frame metric collection across supported Android releases.

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