Do not abort launch tracker when starting with clearing task

Though the launching task becomes empty temporarily after applying
FLAG_ACTIVITY_CLEAR_TASK, it is going to create a new activity on
the task. So keep the tracker to record the draw time from the
new activity.

Bug: 420518895
Flag: EXEMPT bugfix
Test: A trampoline launches the main activity twice with CLEAR_TASK.
      The event log wm_activity_launch_time should still appear.

Change-Id: I919e379df6994010ecaf5de0ace5e8da869e7bfb