Improve unit and functional tests for propagation in case of legacy Activities

[vishweshbankwar]

Related to #2659.

In case of legacy activities (i.e. new Activity()), we run the sampler after the activity is started. When the sampler runs it updates the ActivityTraceFlags property based on the sampling result. However, updating the trace flags on activity does not get reflected on Id if the Id was accessed before sampler runs. This results is incorrect propagation when the propagator uses Id property to set the traceparent header.