Fixed that modem data roaming state was not preserved properly

The modem data roaming state (comparing to the data roaming state
that might be overridden by carrier config or resource overlay)
was saved after it got overridden. Now save it as soon as we get
the state from the modem.

Test: Telephony sanity tests + unit tests
bug: 64155609
Change-Id: I7c54ab2a3fa6d1d6dae7b2c83f5a41bad1faa38b