Prevent potential NPE of checking task drawn for logger

The place is only called from:
1. notifyActivityLaunched (more likely)
   The activity must be launched successfully so ideally its
   task shouldn't be null.
2. notifyVisibilityChanged
   The only caller (setVisibility) has checked non-null
   parent (task).

Still add a simple check for the unknown case.

Bug: 189540136
Test: ActivityMetricsLaunchObserverTests
Change-Id: Iae4a3ad1ae35f1a206d5cf47c7e41e0013aa5077