Exit event could potentially be fired more than once, which will make the app crash because it will attempt to navigate back more than once, leading to "IllegalStateException FragmentManager is already executing transactions"

Closes: #13226
Closes: #13289

Description

Tentative fix for crash:

IllegalStateException: FragmentManager is already executing transactions
    at com.woocommerce.android.ui.products.variations.VariationDetailFragment.setupObservers$lambda$24(VariationDetailFragment.kt:320)
    is Exit -> requireActivity().onBackPressedDispatcher.onBackPressed()
    at com.woocommerce.android.ui.products.variations.VariationDetailFragment.$r8$lambda$6j_9a98nQXRa_5YR2Y_XIxTyzTw(VariationDetailFragment.kt:0)
    at com.woocommerce.android.ui.products.variations.VariationDetailFragment$$ExternalSyntheticLambda14.invoke(VariationDetailFragment:0)
    at com.woocommerce.android.ui.products.variations.VariationDetailFragment$sam$androidx_lifecycle_Observer$0.onChanged(VariationDetailFragment.kt:0)
    at com.woocommerce.android.viewmodel.MultiLiveEvent.observe$lambda$0(MultiLiveEvent.kt:43)
    observer.onChanged(t)

And for the crash:

java.lang.IllegalStateException: FragmentManager is already executing transactions
    at androidx.fragment.app.FragmentManager.ensureExecReady(FragmentManager.java:1937)
    at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1996)
    at androidx.fragment.app.FragmentManager.handleOnBackPressed(FragmentManager.java:862)
    at androidx.fragment.app.FragmentManager$1.handleOnBackPressed(FragmentManager.java:570)
    at androidx.activity.OnBackPressedDispatcher.onBackPressed(OnBackPressedDispatcher.kt:276)

Which has a similar stack trace.

I spent some time trying to replicate the crash but I was unable to. My hypothesis is that the Exit event is triggered simultaneously from VariationDetailViewModel when the user clicks the back button and when loadVariation function fails to load the variation and automatically runs: triggerEvent(Event.Exit). I tried using APIFaker to make different api calls fail when opening variation detail but wasn't able to reproduce the race condition where 2 Exit events are fired at once.

The proposed solution is to basically make sure we unsubscribe from LiveData as soon as the first Exit event is fired, so that any subsequent call to triggerEvent(Event.Exit) will be ignored.

Testing information

Open a product with variations and check that everything works as expected:

• Add new variation works as expected
• Opening variation detail and navigating back and forth works as expected
• Deleting variation works as expected
• Tap on the three dots menu and update several variations in bulk. Check everything works as expected

The tests that have been performed

The above

• I have considered if this change warrants release notes and have added them to RELEASE-NOTES.txt if necessary. Use the "[Internal]" label for non-user-facing changes.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on big (tablet) and small (phone) in case of UI changes, and no regressions are added.