Request starting window first when moving task with shell transition

Both binder calls (add-starting-window, request-transition) from core
to shell will post to wmshell.main thread. And because starting window
controller will post to its wmshell.splashscreen thread, if starting
window is scheduled earlier, these 2 operations can run in parallel.

This aligns the same order as startActivity.

Bug: 235323163
Test: adb shell setprop persist.wm.debug.shell_transit 1; reboot
      Launch app from recents.
Change-Id: Ibcefb4c446217f8a3bdc2e20bfbfce8029d6981b