Add Import/Export (also fix Snackbar layout bug)

I wrote a small feature to allow importing and exporting of the list of names and numbers to a file in primary storage. You can access it through the menu on BlacklistActivity. The default filename is "NoPhoneSpam_blacklist.txt".

Also, the Snackbar that complains about permission problems was being covered by software buttons on my phone and emulator, so I adjusted it. That commit is the first in my patchset, so you could easily merge it if you want the bugfix but not the new feature.