WifiMigration: Add new API's for migrating config store data

The new API's should be modified by the OEM's to convert their existing
config store file to AOSP format. wifi mainline module will
delete/rename the file after migrating data out of it. Subsequent
invocations of these API's should return null.

Bug: 149418926
Test: Compiles
Change-Id: I9ef0986fd38ea5ba9555c7c4652b90cf2ac2fb91