Introduce PersistentDataStore for CdmService

Move functionality for persisting data to an (XML) file on a disk from
CompanionDeviceManagerService.java to a dedicated
PersistentDataStorage.java.
Store the data into getDataSystemCeDirectory instead of deprecated
getUserSystemDirectory. Also make sure the data is transfered to the new
file correctly.

Bug: 1979395
Test: make
Change-Id: I679d8faad7ff8beba99444c3cf057f4a24e66d66