GD-storage: Add LegacyConfigFile

* LegacyConfigFile: Read, save, and delete legacy config on non-volatile storage
* Add serialization function to ConfigCache
* Add change callback input to ConfigCache so that it can notify a
  listener when a persistent device property changed

Bug: 157533831
Test: atest bluetooth_test_gd
Tag: #gd-refactor
Change-Id: Icbedb900b803508c60ee07041903ccb1b23b5544