IssueTracker

The new `OnBackStackChangedListener` callback APIs, `onBackStackChangeStarted` and `onBackStackChangeCommitted` were added as part of the current release but have different timing. The `onBackStackChangeStarted` callbacks are executed before any operations are executed while `onBackStackChangeCommitted` is executed after the operations are executed.

The current timing of `onBackStackChangeCommitted` means that it received fragments that have been detached from the `FragmentManager` and had all of the state deleted, which is not very useful in the callback.

It seems that the timing of `onBackStackChangeStarted` is the correct timing for both callback as currently implemented. We should adjust `onBackStackChangeCommitted` to reflect that. In the future, as fragment integrates with predictive back APIs, we anticipate that the timing of `onBackStackChangeStarted` may change.