WifiOemMigrationHook: Migrate Settings global values

Changes:
a) Add a method to migrate Settings values from platform.
b) Marked all of those settings back to @hide.
c) Also, deprecated couple of public Settings keys which are no longer
used in the wifi stack.

Bug: 148514485
Test: Compiles
Test: TBD: Add unit tests for this class.
Change-Id: I3566cdb53fc997fbb2cc5bceadd865ff8b01539b