Bug 881957 - Implement toast and show toast after successfully change PI...

...N

1. Implement Settings-wide toaster, basically following design of [building block](https://developer.mozilla.org/en-US/Apps/Design/Building_Blocks)
2. Display toast when user successfully change PIN


[Link to Bug 881957](https://bugzilla.mozilla.org/show_bug.cgi?id=881957)