[TabLayoutMediator] Prevent crash autoRefresh disabled mediator

[ryohji]

The TabLayoutMediator constructed with autoRefresh = false is crashed when calling detach. Because pagerAdapterObserver remained null on attach and then on detach adapter.unregisterAdapterDataObserver(pagerAdapterObserver) throws IllegalArgumentException.

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[ryohji]

@googlebot I signed it!

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[cane612]

I think this is a mandatory check if you are reusing TabLayoutMediator instance with condition autoRefresh is false. @ryohji nice contibution

[pekingme]

Thanks for fixing this, LGTM.