Add hideCurrentSnackBar for showing instantly

[jja08111]

If diffrent snackBar is shown, DoubleBackToCloseApp.snackBar is not shown instantly.

So, I want to add Scaffold.of(context).hideCurrentSnackBar() in _handleWillPop Function.

[ghost]

This seems to make total sense.

Please, add a test and take a look at my comment so I can accept your PR :)

[ghost]

@jja08111 I'm not quite sure what you're doing here. This repo already has a workflow to run the tests.

[jja08111]

@hugocbpassos Sorry, I didn't know how to test the PR.

[ghost]

@jja08111 That's not what I meant by "add a test". My apologies.

I want you to write a new test in test/double_back_to_close_app_test.dart which proofs that, when there's another snack-bar currently visible, this widget hides it out and shows up the snack-bar provided to DoubleBackToCloseApp.

If you are not really used to Flutter's testing framework, I can work on those tests for you in the weekend :)

[ghost]

Note to myself: Those commits about the workflow must vanish, so in order to avoid having to rewrite git's history or resubmitting the PR, I should squash before merging.

[hcbpassos]

I fixed some conflicts caused by #16.

[hcbpassos]

I added the test so this should be good to go.

Thanks for opening this PR :)