The onDestroyView method is used to unregister the application observer
from the dialog. However the parent got called first which destroys the
view of the dialog. This could lead to Null Pointer Exceptions when
the listener tries to update the UI of the dialog right after it was
destroyed.
The VolumeControllerDialogFragmentListener was created twice as creation
was triggered on both the down and up event of the volume keys. This has
been changed to only trigger on down events.
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