* Restore AvailabilityStrategy
* Add test for http availability strategy on empty stream from original bug fix 7c17351631
* fix flake errors
* Change CheckStream to use availability strategy
* Refactor test from bug fix
* fix flake errors
* Remove extra helper file from cherry-pick
* Merge tests for default http availability strategy
* turn off HttpAvailabilityStrategy as default (for now) (#21488)
* turn off HttpAvailabilityStrategy as default (for now)
* Update imports accordingly