Use WifiManager for wifi backup/restore

Use the newly exposed WifiManager API's for backing up/restoring wifi
related data. Remove all other wifi related logic from
SettingsBackupAgent.

There are 3 API's exposed by WifiManager for backup/restore:
1. retrieveBackupData - Data to be backed up in new format.
2. restoreBackupData - Restore from the new backup data format.
3. restoreOldBackupData - Restore from the old backup data format.

BUG: 29075035
BUG: 28967335
Change-Id: I2dc379bc36af0a0824ed51fbe9aaebfd0a0114b0