Backport of backup transport whitelist

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 494df791 from N

Change-Id: I9f90e324169a68720d608f74754d284a7e59cf87