Android-SDK - 3.1.0

## 3.1.0
May 13th, 2019

This minor release updates the SDK to use the Optimizely Java SDK 3.1.0 which includes the following:

### New Features:
* Introduced Decision notification listener to be able to record:
* Variation assignments for users activated in an experiment.
* Feature access for users.
* Feature variable value for users.
* Added APIs to be able to conveniently add Decision notification handler (`addDecisionNotificationHandler`) and Track notification handler (`addTrackNotificationHandler`).

### Bug Fixes:
* Feature variable APIs return default variable value when featureEnabled property is false. ([#274](

### Deprecated
* Activate notification listener is deprecated as of this release. Recommendation is to use the new Decision notification listener. Activate notification listener will be removed in the next major release.
* `addActivateNotificationListener`, `addTrackNotificationListener` and `addNotificationListener` APIs on `NotificationCenter`.