NR is lower priority than all other signal strengths

aosp/891956 introduced signal strengths support for NR. In the
method SignalStrength.getPrimary, NR is the lowest priority.
This is against the policy which is in the comment, saying
"prioritizing newer faster RATs".

Bug: 148634820
Test: atest com.android.internal.telephony.SignalStrengthTest
Change-Id: Ia9055c9e8cd8d09cdd3f101750c9f9cea983a9f8