Add in-task activity transition metrics

The in-task activity transition was usually not logged because
the existing metrics focuses on transition with process switch.

So add a new metric to measure the performance for that cases
and it can also provide some hints to evaluate features which
are related to the occurrence.

Bug: 265679829
Test: ActivityMetricsLaunchObserverTests#testInTaskActivityStart

Change-Id: Iec6c862aa11b6aa918a26c0aa89b2d8cc88249ad