Improve Location object.

Add getElapsedRealtimeNano():

Currently Location just has getTime() and setTime() based on UTC time.
This is entirely unreliable since it is not guaranteed monotonic.
There is a lot of code that compares fix age based on deltas -
and it is all broken in the case of a system clock change. System
clock can change when switching cellular networks (and in some
cases when switching towers).

Document the meaning of getAccuracy():
It is the horizontal, 95% confidence radius.

Make some fields mandatory if they are reported by a LocationProvider:

All Locations returned by a LocationProvider must include at the
minimum a lat, long, timestamps, and accuracy. This is necessary
to perform fused location. There are no public API's for applications
to feed locations into a location provider so this should not cause
any breakage.

If a LocationProvider does not fill in enough fields on a Location
object then it is dropped, and logged.

Bug: 4305998
Change-Id: I7df77125d8a64e174d7bc8c2708661b4f33461ea

services/java/com/android/server/LocationManagerService.java

@@ -1532,6 +1532,11 @@ public class LocationManagerService extends ILocationManager.Stub implements Run
             throw new SecurityException("Requires INSTALL_LOCATION_PROVIDER permission");
         }
 
+        if (!location.isComplete()) {
+            Log.w(TAG, "Dropping incomplete location: " + location);
+            return;
+        }
+
         mLocationHandler.removeMessages(MESSAGE_LOCATION_CHANGED, location);
         Message m = Message.obtain(mLocationHandler, MESSAGE_LOCATION_CHANGED, location);
         m.arg1 = (passive ? 1 : 0);
@@ -1588,7 +1593,8 @@ public class LocationManagerService extends ILocationManager.Stub implements Run
 
         // Check whether sufficient time has passed
         long minTime = record.mMinTime;
-        if (loc.getTime() - lastLoc.getTime() < minTime - MAX_PROVIDER_SCHEDULING_JITTER) {
+        long delta = (loc.getElapsedRealtimeNano() - lastLoc.getElapsedRealtimeNano()) / 1000000L;
+        if (delta < minTime - MAX_PROVIDER_SCHEDULING_JITTER) {
             return false;
         }