WifiOemMigrationHook: Rename class & methods to be more generic

This class will hold all the mainline related migration logic.
Currently, includes the config store migration hook. Will be adding the
Settings migration hook here in a future CL.

Bug: 148514485
Test: Compiles
Change-Id: Ie66abbdaa720c0fe0deabc4e3de55e97ccac416e