Bump NetworkStack to system_30

NetworkStack can use system_30 now that system_30 exists.
This change moves the "current" API shims to api30, and adds an api31
shim.

Test: atest NetworkStackTests NetworkStackNextTests, manual (Wifi,
          telephony working)
Bug: 143586229
Change-Id: I37ca0e5a8a5c5620f5fed478648853b668e87f50