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 494df79172 from N
Change-Id: I9f90e324169a68720d608f74754d284a7e59cf87