Various minor issues

[zmuranaka]

There is no analysis_options.yaml included in the project. Just by including this in the project with the default recommended Flutter lints we can see the following issues:

• PinchZoom constructor can be const
• PinchZoom constructor does not expose a Key? parameter
• Recommended to not expose private _PinchZoomState in public API PinchZoom
• _endScrollTimer can use a const Duration

There were two other improvements that I noticed could be made:

• The [resetDuration] parameter has been removed from the PinchZoom constructor but it is still referenced in the documentation comment here.
• The PinchZoom class is missing a dispose method implementation even though _transformationController and _overlayEntry are disposable, and _endScrollTimer is cancelable.