Fix distanceFilter caching for LocationObserver

Summary:
Allow changing distanceFilter after _locationManager has been initialized.

There is 2 reasons for this PR:
- When calling `getCurrentPosition`, `_observerOptions` is possibly not set properly ( as it is set only in `startObserving`), in this case it would have default values so `distanceFilter` will be 0 in this case ( which can be the right value but we should be more explicit about it, `kCLDistanceFilterNone` or `RCT_DEFAULT_LOCATION_ACCURACY`):
<img width="961" alt="screen shot 2016-04-14 at 8 44 09 pm" src="https://cloud.githubusercontent.com/assets/159813/14551465/6aa8791a-0288-11e6-9c98-1687357f8c2a.png">
- Another issue is that `distanceFilter` is cached so it can't be changed afterwards:

```javascript
let options;
options = {
  enableHighAccuracy: true,
  distanceFilter: 20,
};

this.watchId = this.geolocation.watchPosition(
  () => { },
  () => { },
  options,
);

// => sets distanceFilter to 20

this.geolocation.clearWatch(this.watchId);

options = {
  enableHighAccuracy:
Closes https://github.com/facebook/react-native/pull/6987

Differential Revision: D3258956

fb-gh-sync-id: 00a1d1b29d732a54cdc30e20a7a9a2de3dd9b725
fbshipit-source-id: 00a1d1b29d732a54cdc30e20a7a9a2de3dd9b725

Libraries/Geolocation/RCTLocationObserver.m

@@ -130,11 +130,11 @@ RCT_EXPORT_MODULE()
 
 #pragma mark - Private API
 
-- (void)beginLocationUpdatesWithDesiredAccuracy:(CLLocationAccuracy)desiredAccuracy
+
+- (void)beginLocationUpdatesWithDesiredAccuracy:(CLLocationAccuracy)desiredAccuracy distanceFilter:(CLLocationDistance)distanceFilter
 {
   if (!_locationManager) {
     _locationManager = [CLLocationManager new];
-    _locationManager.distanceFilter = _observerOptions.distanceFilter;
     _locationManager.delegate = self;
   }
 
@@ -147,6 +147,7 @@ RCT_EXPORT_MODULE()
     [_locationManager requestWhenInUseAuthorization];
   }
 
+  _locationManager.distanceFilter  = distanceFilter;
   _locationManager.desiredAccuracy = desiredAccuracy;
   // Start observing location
   [_locationManager startUpdatingLocation];
@@ -179,7 +180,7 @@ RCT_EXPORT_METHOD(startObserving:(RCTLocationOptions)options)
     _observerOptions.accuracy = MIN(_observerOptions.accuracy, request.options.accuracy);
   }
 
-  [self beginLocationUpdatesWithDesiredAccuracy:_observerOptions.accuracy];
+  [self beginLocationUpdatesWithDesiredAccuracy:_observerOptions.accuracy distanceFilter:_observerOptions.distanceFilter];
   _observingLocation = YES;
 }
 
@@ -253,7 +254,7 @@ RCT_EXPORT_METHOD(getCurrentPosition:(RCTLocationOptions)options
   if (_locationManager) {
     accuracy = MIN(_locationManager.desiredAccuracy, accuracy);
   }
-  [self beginLocationUpdatesWithDesiredAccuracy:accuracy];
+  [self beginLocationUpdatesWithDesiredAccuracy:accuracy distanceFilter:options.distanceFilter];
 }
 
 #pragma mark - CLLocationManagerDelegate