Making getDataSystemDeDirectory as SystemApi

RescueParty uses this API for getting the list of all the user ids from
files present in this directory.
Since RescueParty is moving to CrashRecovery module, we need
to make it an SystemApi.
More Details: go/v-crashrecovery-module-design-doc

Bug: b/289203818
Test: m

Change-Id: I77786a532763876ee0eb1f14ac8489f42ee8eda6