Correctly skip files that are excluded from restore

BUG: 22957980
If a file was present in the backup but excluded on restore,
it can result in the restored data being corrupted.
Ensure that FullBackup.restoreFile is called with a
null destination, which will result in the file not
being written to disk, but still properly pulled
from the socket.

Change-Id: Iac882a961b76e687654535aec352678486a08c39