Changed in and out animations to follow the Material Design specs.

The Material Design specs appear to have the Snackbar in animation start from about 30% opacity and 60% of the way in (instead of sliding all the way from the bottom), similar to how the keyboard in animation works.

Also renamed animation resource files to snackbar_in and snackbar_out to make it less likely for the parent project to accidentally override the animations. Naming them snackbar_in and snackbar_out also allows people to intentionally override them with custom in and out animations.