Reduce showing starting window from a task trampoline

This skips adding a starting window when a task with starting window
starts another existing task with task fragment organizer while the
task snapshot cannot be used.

That avoids showing consecutive 2 solid color slash starting windows
in a short time with a task transition animation, which looks a bit
redundant. Especially, if the second task is a task fragment host,
then the starting window, the transition may wait for organizer to
response after app has drawn, which could be also wasteful.

Fix: 377973063
Flag: EXEMPT bugfix
Test: LongClickMicMicrobenchmark
Test: An existing TaskA with embedded activity in background.
      Launch a new TaskT. And TaskT launches the TaskA
      with a different intent (allowTaskSnapshot=false)
      in its activity's onCreate. Then wen moving TaskA to front,
      it should not have a starting window.
Change-Id: I795ac2be0c56aaa2544f9b647cbadaf9b6c9fd98