Do not create starting window if app window isDrawn.

The WinAnimator.getShown state will be changed before transition ready,
so there can be a short timing that system can add starting window
during pausing activity -> scheduleIdle -> stop activity while keyguard
locked, because the app window can already report drawn, but the
activity hasn't commit visible. If keyguard going away happen just
between pausing activity -> scheduleIdle, there can have a chance that
starting window stuck on app, becuase app window has already drawn.

Bug: 339586518
Test: steps on the bug
Change-Id: I51eb515fe9bcf73f52f8e8144f54f5b24d9bc044