Overwrite TTFD measurement when API called too early

[stefanosiano]

Description

If the API Sentry.reportFullyDisplayed is called too early, we overwrite the TTFD measurement, but we set it in the TTID span, not the TTFD one.
The problem is that the TTFD span is already finished, and we can't set measurements to already finished spans

[markushi]

Let's allow changing the span/txn values via a private APIs.