[MS57.1] Prepare APIs for data migration utility

This includes:
1. Move PREFIX_* constants to NetworkStatsManager to expose
   in later changes.
2. Rename networkId to wifiNetworkKey.
3. Rename subType to ratType.
4. Replace SUBTYPE_COMBINED with NETWORK_TYPE_ALL
5. Fix lint errors when exposing system api.

Test: TH
Bug: 204830222
Change-Id: I2b7c34958bc59c3225c96f12abba008b83101585