Check null before getting task bounds

The task bounds was used to decide whether multiple launches can
be combined into one event for tracking launch time.

With below sample:
startActivities(new Intent[] {
    new Intent(c, Activity1.class),
    new Intent(c, Activity2.class).setFlags(
        Intent.FLAG_ACTIVITY_CLEAR_TASK
        | Intent.FLAG_ACTIVITY_NEW_TASK) });

When launching Activity2, because Activity1 hasn't real started,
Activity1 will be removed from task directly. Then its task will
be null, that causes mLastLaunchedActivity.getTask() to be null.

If the task is not available, check if the launched activity has
the same uid as the caller uid.

Fix: 276513946
Test: atest ActivityMetricsLaunchObserverTests#testConsecutiveLaunch

Change-Id: I731f0446b62b666b22ba1a6b37364d95efd0a32c