GD-storage: Add StorageModule

* StorageModule is the main entry point of various GD storage mechanisms
* StorageModule is also responsible for reading, writing, and
  periodically syncing disk config with in-memory ConfigCache

Bug: 157533831
Test: atest bluetooth_test_gd
Tag: #gd-refactor
Change-Id: I96daf4169ab91b123d98a119e2d9f92ead7b231d