Nuke mActivitiesWaitingForVisibleActivity

Instead of relying on this brittle architecture we just check
whether the animation has finished. Much cleaner now :-)

Test: Swipe up to recents from Chrome, relaunch in the right
moment. Observe no jumpcut.
Test: Existing lifecycle tests should cover this
Test: android.server.am.lifecycle: All passed!
Bug: 111227410
Change-Id: I83b3dc0e026e842bf260a376ce9a57afd1b01d9f