make non-organized tasks visible when their activity is made visible

This is necessary in a few cases (CTS) where a task is NOT organized but
had its visibility changed within its direct parent. An example of this
is if an alternate home leaf-task HB is started atop the normal home
leaf-task HA: these are both in the Home root-task HR, so there will be a
transition containing HA and HB where HA surface is hidden. If a standard task SA is
launched on top, then HB finishes, no transition will happen since neither home is
visible. When SA finishes, the transition contains HR rather than HA. Since home
leaf-tasks are NOT organized, HA won't be in the transition and thus its surface
wouldn't be shown. Just show is safe here since all other properties will have
already been reset by the original hiding-transition's finishTransaction (we can't
show in the finishTransaction because by then the activity doesn't hide until
surface placement).

Bug: 194112093
Test: atest SplashscreenTests
Change-Id: I885d688b3f505d33e04d9c2e4857928ada1c4fb0