Sysconfig define a whitelist of permitted backup transports Previously any apk bundled in priv-app could insert a backup transport. Reduce risk surface by giving the OEM explicit control over who is allowed to handle backup data. Bug 28406080 Backport of 494df791728f4d42d67e935c327910975993ad29 from N Change-Id: I9f90e324169a68720d608f74754d284a7e59cf87