Memory leak in consumeFlow if fragment is removed before flow is created

[csadilek]

This is similar to #9017, except in this case the leak happens if the fragment is removed before the coroutine (that creates the flow) is launched: https://github.com/mozilla-mobile/android-components/blob/master/components/lib/state/src/main/java/mozilla/components/lib/state/ext/Fragment.kt#L92

We have introduced a check to make sure the fragment isn't destroyed when the flow is produced in #9019, which in theory should have taken care of this, but when fragments are removed their lifecycle is set back to initialized: https://cs.android.com/androidx/platform/frameworks/support/+/androidx-master-dev:fragment/fragment/src/main/java/androidx/fragment/app/Fragment.java;l=2156?q=Fragment ...so it's possible that the fragment is destroyed and removed before this block even launches. We then create a subscription for a fragment that is already removed (its lifecycle.currentState being INITIALIZED) and we will never unsubscribe.

This leak can now be reproduced with the latest changes to SearchDialogFragment in Fenix. STR are mozilla-mobile/fenix#13785.

The simplest solution is to apply the same fix we did for #9017 here as well i.e. create an observer to make sure we notice when the fragment gets destroyed and do nothing / don't create the flow when the coroutine launches.

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