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
81a1c176ba