[Mainline] decouple ServiceState.getDataNetworkType()

Use NetworkRegistrationInfo.getRegistrationState() to
replace serviceState.getDataNetworkType() inside getCombinedServiceState

Bug: 146473838
Test: atest frameworks/base/packages/SettingsLib/tests/robotests/\
src/com/android/settingslib/UtilsTest.java (PASS)

Change-Id: Iddf23778be5d275c76ad01bfbac8f4ed0c655331