Only consider the last launched activity for metrics logger

Otherwise the logger needs to make sure that every visibility changes
belonging to the launch event are handled. Especially when there are
trampoline activities, such as if they are removed without visibility
change, then the launch time and trace may be unexpectedly long.

This simplifies the logic to track undrawn transition info. In general,
the occluded/behind activities are not important to the metrics.

Bug: 204488635
Test: atest ActivityMetricsLaunchObserverTests#testConsecutiveLaunch

Change-Id: I6c7e94a1a43acf642da77a23f400159d78144599