[PB] Avoid resume activity while make paired activities visible.

While make previous activities become visible with LaunchTaskBehind,
there should drive ensureActivitiesVisible after all activities have
set LaunchTaskBehind to true. Otherwise the activities might be resume
accidentally if the first activity isn't top activity.

Flag: EXEMPT bugfix
Bug: 361273875
Test: follow issue description, verify activities won't resume before
commit gesture.

Change-Id: Id9fe10f9046dca07a55479d00c7daf4ceff76c5e