Add PrepareGetSession Construction Log

In recent analysis of metrics, we've noticed this log could provide
valuable information in understanding certain browser calls.
Adding this in so that it can be available in future logs.

Bug: 393172214
Test: Build
Change-Id: If6e9c84e8219a06cad9e0646a80af552856220a6
Flag: android.credentials.flags.fix_metric_duplication_emits