Storage: Only save when storage has changed

After the file is loaded from storage, each change to the storage calls
SaveDelayed through the persistent_config_changed_callback.  If that has
been called, write the config to disk when shutting down the storage
module, otherwise do nothing.

Bug: 260006855
Test: atest bluetooth_unit_test_gd
Change-Id: I7b653032aa3398d2f95236065572ba7fe733ed9c