WifiBackupRestore: Set hidden flag from old backup

Add the missing |hiddenSSID| element parsing from old backups and add
unit-tests to validate hidden network backup/restore.

While there,
Add input parameter validation in all the public methods.

BUG: 29075035
Change-Id: I60c8acea17a532e7d5810d1096fbefc647614027
TEST: Unit tests
2 files changed
tree: 868f1c3440cca920d4833305d1bad1ae552dad90
  1. service/
  2. tests/