Bump android tools gradle plugin to 0.12.+, and use the robolectric-gradle-plugin 0.12.+

There was an issue with packaging MozStumbler in fdroid due to build.gradle using 0.12.2
and android/build.gradle using 0.10.
Merge pull request #1123 from keverets/robolectric-gradle-plugin-update

Bump android tools gradle plugin to 0.12.+, and use the robolectric-gradle-plugin 0.12.+