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 branch 'master' of https://github.com/facebook/facebook-android-sdk into develop
Update minSdkVersion, use android.app.Fragment for Sessions