Fix race condition generating READY and NOT_SUPPORTED statuses.

The race condition only affects when the client registers for several (all) location listeners.
And the side efects are benign: only the measurement and navigation message status are incurrectly
being sent to the application, but there are no crashes or any real data from GPS being
misscommunicated.
Also:
- cache the last reported status to filter sending notifications when no changes have occurred
- do some cleanup and refactoring in the code changed

Change-Id: I0692e6b70847dc1ee092d7a05a2c6ba3cd9fa147

location/java/android/location/GpsNavigationMessageEvent.java

@@ -36,18 +36,18 @@ public class GpsNavigationMessageEvent implements Parcelable {
      * The system does not support tracking of GPS Navigation Messages. This status will not change
      * in the future.
      */
-    public static int STATUS_NOT_SUPPORTED = 0;
+    public static final int STATUS_NOT_SUPPORTED = 0;
 
     /**
      * GPS Navigation Messages are successfully being tracked, it will receive updates once they are
      * available.
      */
-    public static int STATUS_READY = 1;
+    public static final int STATUS_READY = 1;
 
     /**
      * GPS provider or Location is disabled, updated will not be received until they are enabled.
      */
-    public static int STATUS_GPS_LOCATION_DISABLED = 2;
+    public static final int STATUS_GPS_LOCATION_DISABLED = 2;
 
     private final GpsNavigationMessage mNavigationMessage;