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: I405b49daee8c576584575c3e46877cc97632d8c6