Hygiene: upgrade Android Plugin for Gradle to v2.3.1
Hygiene: upgrade Android Plugin for Gradle to v2.3.1

> This is a minor update to Android plugin 2.3.0 that fixes an issue
  where some physical Android devices did not work properly with Instant
  Run (see Issue #235879).

https://developer.android.com/studio/releases/gradle-plugin.html#revisions

Change-Id: Id4b1a3338c86d8430bc741e64dfc6e23616dbbf3

wikimedia/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,wikimedia/apps-android-wikipedia,wikimedia/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,wikimedia/apps-android-wikipedia