Fix weird animation when switching back to split pair from recents

Since Launcher indexes TaskView with leaf task ids directly. Collects
opening leaf tasks only to make sure Launcher can identify the proper
TaskView to animate in while finishing the recents transition.

Bug: 223396206
Test: adb shell setprop persist.wm.debug.shell_transit 1 && adb reboot,
      enter split mode, switch back and forth between the split pair and
      normal fullscreen tasks in overview. Verified both of the
      splitting tasks animate in together.
Change-Id: Icec4e3376d7127f6a9ab2820057a2bba63539c3f