AML: Abort launching metrics if no activities will be drawn

Otherwise it may report a large launch time on an invisible
activity that depends on when to trigger next transition.

Bug: 123355661
Test: atest ActivityMetricsLaunchObserverTests# \
            testOnActivityLaunchCancelled_finishedBeforeDrawn

Change-Id: I6f936929a5ed6ade0e6350d9fde8229d6aaad558