Discard invisible pending draw activity if transition starts

A launching activity may never have visibility set. If another
drawn activity in the same launch sequence can trigger to start
transition, the launch event should still be done successfully.

E.g. warm launch from notification with a trampoline activity.
The trampoline activity keeps the initial invisible state
because lockscreen hasn't gone. And only the target activity will
be set to visible and drawn to start transition.

Fixes: 189882294
Test: atest ActivityMetricsLaunchObserverTests# \
      testDoNotCountInvisibleActivityToBeDrawn

Change-Id: I1cd5d016c70e5f7865d3d44c8d7ceaa97d5a34ea