Fix Android's zIndex SymbolOption (#312)

Before this commit, passing the `zIndex` SymbolOption would be inert as
the android implementation thought this option was called
'symbolSortKey'. This commit corrects that so that the Android zIndex
implementation works again.

android/src/main/java/com/mapbox/mapboxgl/Convert.java

@@ -374,7 +374,7 @@ static void interpretSymbolOptions(Object o, SymbolOptionsSink sink) {
     if (geometry != null) {
       sink.setGeometry(toLatLng(geometry));
     }
-    final Object symbolSortKey = data.get("symbolSortKey");
+    final Object symbolSortKey = data.get("zIndex");
     if (symbolSortKey != null) {
       sink.setSymbolSortKey(toFloat(symbolSortKey));
     }