[story_shell] Add onHandlingBackGesture for session shell coordination

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This is a reland of f51521954ece1c08022194e918b290376afef3b3

Original change's description:
> [story_shell] Add onHandlingBackGesture for session shell coordination
>
> Adds a signal Story Shell and Session Shell can use to coordinate
> gesture handling on Story state that indicates whether the Story Shell
> wants to use back gestures to dismiss mods, or can defer to Session
> Shell for dismissing stories.
>
> Whenever the handling state changes, onHandlingBackGesture is triggered
> which precipitates a change in story state.
>
> Test: manual
>
> Change-Id: I94f51e048d968ac187f017cf6ba3d9aba16f5436

Change-Id: I4cd11ba904f531c7e96958c5c6877decd9f6506f
2 files changed