Update Gradle Android plugin version, fixes T802

Summary:
  - Upgrade Gradle Android plugin to v0.13
  - Upgrades gradle wrapper to v2.1
  - Remove lpreview declaration in the build script.
  - Upgrade roboelectric plugin to v12 to work with the new gradle android plugin.

Test Plan:
  - Build app
  - See it build successfully

Reviewers: dkobia

Subscribers: rjmackay

Maniphest Tasks: T802

Differential Revision: https://phabricator.ushahidi.com/D356