* With non-static ngUpgrade apps, callbacks to `whenStable` were being invoked with the wrong
  context
* With non-static ngUpgrade apps, `resumeBootstrap` was being run outside the NgZone
* Remove redundent `whenStableContext` variable

Neither of the first two problems were actually causing bugs (as far as I know), but they *might*
have caused problems in the future.

Inspired by https://github.com/angular/angular/pull/12910, but for non-static apps.