Clean up the settings activity. Because we are targeting sdk > 11, we
can remove a lot of cruft.

Add some preference validation. Add tests for validation. Note: this
only validates uri syntax, not if the hosts actual exist. That will come
later as a button (or something).

Tested:
  ./gradew clean test
Merge pull request #91 from nightscout/tr.prefvalidation

Clean up the settings activity. Because we are targeting sdk > 11, we