Execute transition by starting window only if visible requested

Otherwise the activity may not have been added as opening app for
transition, which causes the animation to be skipped. Such as if the
previous activity doesn't need to pause or take long time to pause,
then its visible-requested is still false.

Bug: 161781889
Test: Cold launch Settings to freeform:
      adb shell am start -n "com.android.settings/.Settings " \
      --windowingMode 5
      There should be transition animation.
Change-Id: If339b1e9e5f004087dcbf2be3c8ab7a9f5524c5b