IpConfigStore: Changes for parsing old wifi backup

Some changes in IpConfigStore to parse old backup data. The old backup
data contained a raw byte dump of the ipconfig.txt. So, make changes in
IpConfigStore to parse the inputstream from backup data into
IpConfiguration objects.

Changes in the CL:
1. Make the |readIpAndProxyConfigurations| method static so that
it can be used as a utility for parsing the backup data.
2. Create a new version of |readIpAndProxyConfigurations| method to
accept an input stream parameter.
3. Make |writeConfig| method static so that it can be used for
unit-testing the backup migration logic.

BUG: 29075035
Change-Id: Ic074952d9f9ef143089a371e6c527470cb1c229c