Expose NR State string method for use in RadioInfo

Test: build
Bug: 160825855
Change-Id: Ie709cad1c94ad204c46a559d316f45da16c3541c
diff --git a/telephony/java/android/telephony/NetworkRegistrationInfo.java b/telephony/java/android/telephony/NetworkRegistrationInfo.java
index 93fbb00..6a67ad2 100644
--- a/telephony/java/android/telephony/NetworkRegistrationInfo.java
+++ b/telephony/java/android/telephony/NetworkRegistrationInfo.java
@@ -572,7 +572,8 @@
         return "Unknown reg state " + registrationState;
     }
 
-    private static String nrStateToString(@NRState int nrState) {
+    /** @hide */
+    public static String nrStateToString(@NRState int nrState) {
         switch (nrState) {
             case NR_STATE_RESTRICTED:
                 return "RESTRICTED";