Expose option for distance filtering on location updates.

Summary:
My original implementation involved creating a `RCT_ENUM_CONVERTER` with `CLLocationAccuracy` on iOS and a Hashmap on Android that would convert `string` values to `doubles` for distance filtering.

I got this to work just fine but realized that I made things more complicated than they needed to be and simplified everything by just have the option be a decimal value (in meters) that works both for iOS and Android.

The only thing i'm not sure about is if we can set arbitrary values for CLLocationManager's distance filter.
nicklockwood  Any idea?
Closes https://github.com/facebook/react-native/pull/5563

Reviewed By: svcscm

Differential Revision: D2908250

Pulled By: nicklockwood

fb-gh-sync-id: d83c12b3ce7c343f413749a2cd614b3bf04d6750

Libraries/Geolocation/Geolocation.js

@@ -26,6 +26,7 @@ type GeoOptions = {
   timeout: number;
   maximumAge: number;
   enableHighAccuracy: bool;
+  distanceFilter: number;
 }
 
 /**
@@ -68,7 +69,7 @@ var Geolocation = {
 
   /*
    * Invokes the success callback whenever the location changes.  Supported
-   * options: timeout (ms), maximumAge (ms), enableHighAccuracy (bool)
+   * options: timeout (ms), maximumAge (ms), enableHighAccuracy (bool), distanceFilter(m)
    */
   watchPosition: function(success: Function, error?: Function, options?: GeoOptions): number {
     if (!updatesEnabled) {

Libraries/Geolocation/RCTLocationObserver.m

@@ -31,6 +31,7 @@ typedef struct {
   double timeout;
   double maximumAge;
   double accuracy;
+  double distanceFilter;
 } RCTLocationOptions;
 
 @implementation RCTConvert (RCTLocationOptions)
@@ -38,10 +39,15 @@ typedef struct {
 + (RCTLocationOptions)RCTLocationOptions:(id)json
 {
   NSDictionary<NSString *, id> *options = [RCTConvert NSDictionary:json];
+
+  double distanceFilter = options[@"distanceFilter"] == NULL ? RCT_DEFAULT_LOCATION_ACCURACY
+    : [RCTConvert double:options[@"distanceFilter"]] ?: kCLDistanceFilterNone;
+
   return (RCTLocationOptions){
     .timeout = [RCTConvert NSTimeInterval:options[@"timeout"]] ?: INFINITY,
     .maximumAge = [RCTConvert NSTimeInterval:options[@"maximumAge"]] ?: INFINITY,
-    .accuracy = [RCTConvert BOOL:options[@"enableHighAccuracy"]] ? kCLLocationAccuracyBest : RCT_DEFAULT_LOCATION_ACCURACY
+    .accuracy = [RCTConvert BOOL:options[@"enableHighAccuracy"]] ? kCLLocationAccuracyBest : RCT_DEFAULT_LOCATION_ACCURACY,
+    .distanceFilter = distanceFilter
   };
 }
 
@@ -128,7 +134,7 @@ RCT_EXPORT_MODULE()
 {
   if (!_locationManager) {
     _locationManager = [CLLocationManager new];
-    _locationManager.distanceFilter = RCT_DEFAULT_LOCATION_ACCURACY;
+    _locationManager.distanceFilter = _observerOptions.distanceFilter;
     _locationManager.delegate = self;
   }