Fix a race condition in the SentryFramesTracker and
SentryDelayedFramesTracker that sometimes leads to frame delay durations
longer than the queried interval of start and end time. This is fixed by
moving the previousFrameSystemTimestamp down to the
SentryDelayedFramesTracker so we can adequately synchronize it without
acquiring extra locks on the main thread.

Add SentryFramesDelayResult containing framesContributingToDelayCount,
which is the count for the frames that contributed to the frames delay
count. This is required for GH-3492.