Clear invisible undrawn activities when reporting drawn

There may have initializing activities put in the same launch stack
without state and visibility change. The case happens when using
startActivities to start multiple activities at once.

Without clearing the invisible pending draw activities, the launch
event cannot end to report launch time and stop trace.

Bug: 196917563
Bug: 192043869
Test: ActivityMetricsLaunchObserverTests#testConsecutiveLaunch
Change-Id: If511a1526b803455a967f6bc1358a23e15a4234d