Add the app back to mOpeningApps if transition is unset but ready.

This means a rotation is started while we have a transition pending,
and startFreezingDisplayLocked() called mAppTransition.freeze() to
cancel the transition by setting it to TRANSIT_UNSET but ready
immediately. Screen unfreeze will wait for mOpeningApps to empty.
handleAppTransitionReadyLocked() will remove the app from
mOpeningApps once it's drawn, and allow screen to unfreeze.

We don't add the app to mOpeningApps if the transition is neither
set nor ready. We don't have any transtition and the app won't be
removed from mOpeningApps.

bug: 27784481
bug: 27391256
Change-Id: Id2c0759732593121769c402ae0c6edde3ebc7dc6