Fix majority of Android Studio warnings

[johnmaguire]

• Set minSdkVersion to 26 which addresses two problems:
  1. File locking relies on java.nio.file APIs which do not exist until API level >= 26.
  2. Android 6 and Android 7 (API level 23 and 24) do not support the Let's Encrypt TLS certificate served by the DN API.
• Upgraded Kotlin, Gradle, Gson, and targetSdkVersion.
• Fixed a couple spots where code is deprecated.
• Fixed a reference to the wrong variable (vpnIp -> addr).
• Removed some unnecessary imports and semicolons.
• Kotlin style stuff (var vs. val, avoid getters, etc.)