Revert "Add public fields to NetworkScore and make it @SystemApi"

This reverts commit 511f7e3792.

Bug: 113554781
Test: FrameworksNetTests FrameworksTelephonyTests FrameworksWifiTests
Change-Id: Ie1e84bd2a16f088307b195f924f4e49335cbe367

tests/net/java/com/android/server/connectivity/LingerMonitorTest.java

@@ -353,7 +353,8 @@ public class LingerMonitorTest {
         NetworkCapabilities caps = new NetworkCapabilities();
         caps.addCapability(0);
         caps.addTransportType(transport);
-        NetworkScore ns = new NetworkScore.Builder().setLegacyScore(50).build();
+        NetworkScore ns = new NetworkScore();
+        ns.putIntExtension(NetworkScore.LEGACY_SCORE, 50);
         NetworkAgentInfo nai = new NetworkAgentInfo(null, null, new Network(netId), info, null,
                 caps, ns, mCtx, null, null /* config */, mConnService, mNetd, mDnsResolver, mNMS,
                 NetworkProvider.ID_NONE);