Facebook Android SDK 3.15.0

Summary: Facebook Android SDK 3.15.0 (see https://developers.facebook.com/docs/android/change-log-3.x/)

Test Plan: Built ZIP

Reviewers: clang

Reviewed By: clang

Subscribers: platform-diffs@lists, dmmiller

Differential Revision: https://phabricator.fb.com/D1380454
Merge tag 'sdk-version-3.15.0'

SDK 3.15.0
Updated version to 3.15.0-SNAPSHOT.