Restore old BackstackReader.isInBackstack() implementation once AndroidX Fragment 1.2.1 is released

[lukaszkalnik]

There is a bug in AndroidX Fragment 1.2.0 which crashes the app when changing orientation in fragments or when switching fragments. This is because of our BackstackReader.isInBackstack() hacky implementation. https://github.com/sockeqwe/mosby/issues/318#issuecomment-577660091

A temporary workaround has been implemented for this bug here: https://github.com/grandcentrix/ThirtyInch/pull/205 However, the workaround uses reflection so our previous hack for this problem has to be restored once AndroidX Fragment 1.2.1 gets released (this release will fix the current bug): https://issuetracker.google.com/issues/148189412

[wietsebuseyne]

androidx:fragment 1.2.2 is out, so the old implementation should work again :-) Thank you for your work BTW