Make metrics proto RAT consistent with ServiceState.

Number 20 was defined as RIL_RADIO_TECHNOLOGY_NR, which is meant to be
used by SA. 21 has no corresponding definition in the framework and
there is no number for NSA, but rather the RAT should be LTE and the
NrState should be NR_STATE_CONNECTED.

The proto enum names are not used in any way in the telephony
framework.

Bug: 160616371
Test: build, atest FrameworksTelephonyTests, dumpsys (w/o NR device or SIM)
Change-Id: I5179387120887d53929e0fe82a302e69322ad56d