Summary:
Merge pull request #665 by adamretter
Exposes BackupEngine from C++ to the Java API. Previously only BackupableDB was available
Test Plan: BackupEngineTest.java
Reviewers: fyrz, igor, ankgup87, yhchiang
Reviewed By: yhchiang
Subscribers: dhruba
Differential Revision: https://reviews.facebook.net/D42873
