Calling show() consecutively

[Ting Yik]

Apart from #26, when calling show() consecutively without dismiss(), e.g. showing "Updating database..." and if the result immediately returned to show "Database updated!", due to the way SnackbarManager works, it can trigger the callback differently than expected. Typically the call will be:

snackProgressBar1.show() 
snackProgressBar2.show()
snackProgressBar1.onDismissed()     // Due to snackProgressBar1 immediately dismissed before showing
snackProgressBar1.onShown()
snackProgressBar2.onShown()

This can mess up user operation if there are operations pending in the callback.

[Ting Yik]

b4f3f98 makes sure that BaseCallback.onShown() is called first i.e. snackProgressBar1 has finished appear animation before showing the next in queue.