Unify zIndex property type to other overlays

All overlays (like polygon, circle, polyline) were using the type int
instead of double, except for TileOverlay.
We changed it to have the same interface and be compatible with iOS,
which require an int type.

packages/google_maps_flutter/lib/src/tile_overlay.dart

@@ -79,7 +79,7 @@ class TileOverlay {
 
   /// The tile overlay's zIndex, i.e., the order in which it will be drawn where
   /// overlays with larger values are drawn above those with lower values
-  final double zIndex;
+  final int zIndex;
 
   /// The visibility for the tile overlay. The default visibility is true.
   final bool visible;
@@ -99,7 +99,7 @@ class TileOverlay {
     TileOverlayId tileOverlayId,
     bool fadeInParam,
     double transparencyParam,
-    double zIndexParam,
+    int zIndexParam,
     bool visibleParam,
     int tileSizeParam,
   }) {

packages/google_maps_flutter/test/fake_maps_controllers.dart

@@ -365,7 +365,7 @@ class FakePlatformGoogleMap {
       final String tileOverlayId = tileOverlayData['tileOverlayId'];
       final bool fadeIn = tileOverlayData['fadeIn'];
       final double transparency = tileOverlayData['transparency'];
-      final double zIndex = tileOverlayData['zIndex'];
+      final int zIndex = tileOverlayData['zIndex'];
       final bool visible = tileOverlayData['visible'];
 
       result.add(TileOverlay(