Add Doc and nullability to LocationManager#getNetworkProviderPackage

Test: Built
Bug: 116798569
Change-Id: Iead34b0f93e4f5331ba49ec0d170f1ec0db0d4c3

location/java/android/location/LocationManager.java

@@ -2380,10 +2380,12 @@ public class LocationManager {
     }
 
     /**
+     * Return the package that implements the {@link #NETWORK_PROVIDER} functionality.
+     *
      * @hide
      */
     @SystemApi
-    public String getNetworkProviderPackage() {
+    public @Nullable String getNetworkProviderPackage() {
         try {
             return mService.getNetworkProviderPackage();
         } catch (RemoteException e) {